CCELENTERATA. 27 



and nearly regular, for the whole of the oosperm divides, and 

 the resultant cells (blastomeres) are of about the same size. At 

 the end of cleavage the embryo is a hollow sphere or blastula 

 (blastosphere), the cavity of which is known as the blastocoele 

 (segmentation cavity). Cells are now budded off into the blasto- 

 coele from the attached side of the blastula, until the embryo 

 becomes a solid mass, consisting of an external layer of cells, the 

 ectoderm (epiblast) covering an internal cellular core, the endoderm 

 (hypoblast). At the same time a double protective investment, 

 the outer layer of which is firm and chitinous, is secreted by the 

 ectoderm. The changes above described occupy about four days, 

 and, after their completion, the embryo falls from the ovary into 

 the mud, where its development is slowly completed during the 

 winter months. 



The further changes consist in the formation of a digestive 

 cavity by absorption of some of the endoderm cells, while the 

 remaining ones constitute a layer surrounding this cavity. The 

 ectoderm at the same time becomes differentiated, and the meso- 

 glcea makes its appearance. Later on the outer membrane is 

 ruptured by increase in size and elongation of the embryo, the 

 mouth is then formed as a perforation, tentacles grow out as hollow 

 processes of the body-wall, and the inner membrane is cast off. 



The development of Hydra has been investigated by several writers, 

 whose accounts are conflicting. The brief outline given above is probably 

 correct in the main. 



Further remarks on Hydra: 



It must be remembered that although Hydra is a simple and readily 

 obtainable type of the lower multicellular animals, it is by no means a 

 typical example of the group, i.e., the HydrOZOa, to which it belongs. 

 These are for the most part colonial, the colonies being produced by bud- 

 ding. If, in Hydra, the buds, instead of becoming detached, remained 

 united together, something resembling one of these " hydroid zoophytes" 

 would be produced. These colonial forms give rise, in a large number of 

 cases, to free-swimming sexual individuals, which are somewhat umbrella- 

 shaped, and are popularly known as "jelly-fish," technically as "medusae." 

 This is a good example of "alternation of generations," where asexual and 

 sexual stages alternate in the life history of the same form. Thus : A, an 

 asexual hydroid colony gives rise to S, sexual medusa;, which again pro- 

 duce A, and so on, indefinitely. By taking a selected series of forms it can 

 be shown that the sexual organs of Hydra are probably morphologically 

 equivalent (homologous) to medusae. From (1) forms with free swimming 

 medusas, we can pass to (2) forms with medusa-like buds, and thence to 

 (3) forms with sexual buds obscurely medusa-like, lastly to (4) Hydra, 

 with sexual buds as mere knobs of ectoderm. If this reasoning be correct, 

 Hydra is a degenerate form i. e, derived from ancestors more highly differ- 

 entiated than itseLf. 



