CNIDARIA 53 



Concerning the development of Htjdrocorallla there are as yet only 

 scattered observations. Moseley (No. 44) found in the Stylasteridre 

 well-developed planulas within the gonophores. The larvae of Millepora 

 also appear to become free at this stage. In this case the very small 

 eggs, with scanty yolk, pass through the first stages of development in the 

 entoderm of the coenosarc, where they are often attached by a stalk-like 

 Pseudopodium to the supporting lamella. Subsequently they migrate 

 into the entoderm of the basal plate of the gastrozoöid, where they 

 develop into planulae. It is remarkable that the early development is 

 here accompanied by a considerable increase in the number of the em- 

 bryonic nuclei, but without distinct cleavage (Hickson, No. 30, and Nos. 

 VI. and VII., Appendix to Literature on Hydroidea). 



Hypogenetic Medusae. — In the groups of the Tracho- 

 medusai and Narcomedusm the alternation of generations, con- 

 sisting in the regular recurrence of polyps and medusae, is 

 wanting, since in these instances the polyp-generation 

 appears to be suppressed. The young medusge are developed 

 from the egc^ directly, but in many cases still have to pass 

 through a metamorphosis. In the Cuninas, hovpever, there is 

 a secondary introduction of alternation of generations, the 

 larva developed from the egg giving rise by a process of bud- 

 ding to medusas (parasitic bud-spikes [Knospe^iäliren'] of the 

 Cuninas). 



The development of the e^g of the Gerijonidfe has been 

 studied in several species by MktsChnikoff (Nos. 42 and 12), 

 Fol (No. 25), and Brooks (No. 17). The Greryonid egg, 

 which is expelled from the mother's mouth, is surrounded by 

 a mucilaginous envelope, and shows a distinct separation into 

 agranular ectoplasm and a foam-like, clearer endoplasm. By 

 total and equal cleavage there are produced two, four, eight, 

 etc., blastomeres, in which a supei-ficial ectoplasmic and an 

 inner endoplasmic portion can be recognized (Fig. 20 A). In 

 the sixteen-cell stage there is usually to be seen a cleavage 

 cavity (Fig. 2,0 A, h) produced by separation of the blasto- 

 meres. If this represents the blastula-stage, the following- 

 stages inaugurate the formation of the entoderm, which, 

 according to the concurrent testimony of the investigators 

 mentioned above, takes place by means of a so-called delamina- 

 tion process. 



By a transverse division of each of the cleavage spheres 



