52 



EMBRYOLOGY 



Fig. 19.— Actinula of riibidana (after 

 CiAMiciAN). m, incipient oral tentacles. 



tenellum Hasunn (No. 27) and in Campanularia caliculata (?) Metschxi- 

 KOFF (No. 12) have observed the formation, of the entoderm by immigra- 

 tion, this type appears to be more wide-spread among the hydroid polyps 

 than has been assumed hitherto. 



After the cleavage cavity is completely filled with entoderm cells, a 



double egg-membrane is secreted, 

 an inner germ envelope and an 

 outer, harder chitinous shell. 

 Whereas, according to Kleinen- 

 BERG and KoROTNEFF, the ectoder- 

 mic layer is wholly consumed in 

 the formation of the latter, Ker- 

 scHNER was able to show that the 

 ectoderm persists. The egg now 

 detaches itself from the body of the 

 mother and sinks to the bottom, 

 ■the mass of entodermic cells, by 

 the formation of numerous con- 

 necting cords of protoplasm and interstices between them, then assumes an 

 appearance similar to that of connective tissue (Kerschner) ; and then 

 the gastral cavity makes its appearance in this mass. Finally, the outer 

 (chitinous) shell of the germ goes to pieces ; and the embryo, still enclosed 

 in the inner envelope, emerges from it. The tentacles now arise as 

 evaginations of the wall, and the mouth-opening is formed by a breaking 

 through of the wall at a place which corresponds to the vegetative pole 

 (Kerschner), so that after the dissolution of the inner membrane the 

 young Hydra becomes free in a form that resembles an actinula. 



Statements concerning the laws which govern the appearance of the 

 tentacles in Hydra have thus far disagreed. Kleinenberc. maintains 

 that all the tentacles appear at the same time, whereas Korotneff asserts 

 that they arise in pairs, as Mebeschowsky has affirmed for buds. While 

 Jung (No. 31) was unable to recognize in the latter any definite law, 

 Haacke (No. 28) believed that he had observed that the Hydras, apart 

 from the green form, were divisible into two species, which he dis- 

 tinguished as H. Tremblyi and H. Eoeselii. On the buds of the former 

 all (six) tentacles arise simultaneously ; the appearance of the tentacles 

 on the buds of H. Koeselii, on the contrary, discloses a definite orienta- 

 tion in relation to the maternal organism, inasmuch as (the insertion of 

 the bud being perpendicular) the two tentacles first to ajipear lie in a plane 

 dividing the maternal organism transversely, while the third sprouts out 

 in a plane perpendicular to the first and toward the oral side of the 

 mother, the fourth opposite the latter, etc. Such examples prove 

 that in stock-forming radiate animals the bilateral symmetry of the bud 

 is caused by its relation to the parent organism. We must therefore 

 attribute the bilateral structure of many Coelenterates (Anthozoa, young 

 Scypho-polypi) to stock-formation. 



