: The Works of Francis Maitland Balfour Volume 2 (of 4) A Treatise on Comparative Embryology: Invertebrata by Balfour Francis M Francis Maitland Foster M Michael Sir Editor Sedgwick Adam Editor - Zoology; Embryology

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or hypoblast is mainly formed of large nutritive cells. From these cells an epithelial hypoblastic layer becomes secondarily differentiated, the exact relations of which differ somewhat in the various types. The nutritive cells themselves do not appear to become directly converted into the permanent hypoblastic tissues. The development of the adult from the planula commences by the thickening of the epiblastic layer, usually at one pole , and the formation at this pole of a series of bud-like structures , which become converted into the hydrophyllia, nectocalyces etc. The main oral part of the planula becomes generally converted into the polypite, though in some instances it remains as a yolk-sack, and only secondarily gives rise to a polypite.

The difficulty of deciding this point on embryological evidence depends on the fact that ontologically a tentacle and a true bud arise in the same way, viz. as papilliform outgrowths containing prolongations of both the primitive germinal layers. The balance of evidence is nevertheless in my opinion in favour of regarding the Siphonophora as compound stocks, and the views of Claus on this subject appear to me the most satisfactory.

The most primitive condition is probably that like Physophora in an early stage with an hydrophyllium enclosing a polypite . In this condition the whole larva may be compared to a single Medusa in which the primitive hydrophyllium represents the umbrella of the Medusa, and the polypite the manubrium. The tentacle which appears so early is probably not to be regarded as a modified zooid, but as a true tentacle. The absence of a ring of tentacles is correlated with the bilateral symmetry of the Siphonophora.

The primitive zooid of a Siphonophora stock is thus a Medusa. Like Sarsia and Wilsia this Medusa must be supposed to have been capable of budding. The ordinary nectocalyces by their resemblance to the umbrellas of typical Medusae are clearly such buds of the medusiform type. The same may be said of the pneumatophore, which, as pointed out by Metschnikoff, is identical in its development with a nectocalyx. Both are formed by a solid process of epiblast in which a cavity--the cavity of the nectocalyx or pneumatocyst--is eventually hollowed out. Around this there appears a double layer of hypoblast containing a prolongation of the gastrovascular cavity; and this is in its turn enclosed by a layer of epiblast which forms the covering of the convex surface of the nectocalyx and the external epiblast of the pneumatophore.

The generative gonophores are clearly also zooids, and the hydrophyllia are probably a rudimentary form of umbrella. In many cases the hydrophyllium of the primitive polypite is absent. In such instances it is necessary to suppose that the umbrella of the primitive zooid of the whole colony has become aborted. Leuckart originally took a somewhat different view from the above in that he regarded the starting point of the Siphonophora to be a compound fixed Hydrozoon stock, which became detached and free-swimming.

Acraspeda. The embryonic development of several of the forms of the Acraspeda has been investigated by Kowalevsky and Claus . Their observations seem to point to an invaginate gastrula being characteristic of this group.

I use this term for the group, often known as the Discophora, which includes the Pelagidae, Rhizostomidae, and Lucernaridae.

At the point of attachment there is developed a peculiar pedal disc, and around the mouth there appears a fold of epiblast which gives rise to an oral disc . Two tentacles first make their appearance, but one of these is primarily much the largest, though eventually the second overtakes it in its growth. A second pair of tentacles next becomes formed, giving to the larva a 4-radial symmetry. Between these four new tentacles subsequently sprout out, and in the intermediate planes four ridge-like thickenings of the hypoblast, projecting into the cavity of the stomach, make their appearance. They imperfectly divide the stomach into four chambers, to each of which one of the primary tentacles corresponds; they may be regarded as homologous with the mesenteries of the Actinozoa. The number of tentacles goes on increasing somewhat irregularly up to sixteen. All the tentacles contain a solid hypoblastic axis. Muscular elements are developed from the epiblast.

With the above changes the so-called Hydra tuba or Scyphistoma form is reached . The peculiar strobilization of this form is dealt with in the section devoted to the metamorphosis.

Aurelia is stated by Kowalevsky to develop in the same way as Cassiopea; and the one stage of Rhizostoma observed is that in which it has a gastrula form.

In Pelagia the ovum directly gives rise to a form like the parent. The segmentation and the invagination take place nearly as in Cassiopea, but the archenteric cavity is relatively much smaller, and the large space between it and the epiblast becomes filled with the gelatinous tissue which forms the umbrella. The blastopore does not appear to close but to become directly converted into the mouth. As in Cassiopea the larva takes a somewhat four-sided pyramidal form. The mouth is placed at the base. The pyramid becomes subsequently flatter, and at the four corners four tentacles grow out which increase to eight by division. The flattening continues till the larva reaches a form hardly to be distinguished from the Ephyra resulting from the strobilization of the fixed Scyphistoma form of other Acraspeda.

Alcyonidae. In the Alcyonidae the segmentation appears always to lead to the formation of a solid morula, which becomes a planula by delamination. The true enteric cavity is formed by an absorption of the central cells, but the axial portion of the gastric cavity and mouth are formed by an epiblastic invagination.

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