24 o COMPARATIVE ANATOMY 



All this while the embryo remains attached to the parent, 

 and has no external protection save the thin gelatinoid coat 

 described above. The ectoderm cells now secrete a thick 

 external chitinous envelope covered all over with thorny pro- 

 cesses, and one result of their activity is that the deutoplasts 

 which they contained are absorbed and disappear. The 

 endoderm cells are still full of deutoplasts, and so the two 

 layers are still more sharply marked off from one another. 

 After the thick chitinous shell is formed, a second thin, homo- 

 geneous elastic envelope is formed within it, and about this 

 time the embryo is detached from the parent and falls to the 

 bottom. It is now a solid aggregate of cells, divided into an 

 outer layer with finely granular protoplasm, and an inner mass 

 stuffed with reserve material in the form of deutoplasts. The 

 1 whole is enclosed by two protective envelopes. In this con- 

 dition the embryo passes into a resting stage and undergoes 

 no change for several weeks. The first sign of further 

 development is the appearance of a number of small rounded 

 cells between the inner ends of the ectoderm cells. These are 

 the interstitial cells. There is as yet no trace of a mesogloea. 

 A further resting period follows the development of the 

 interstitial cells, and then the embryo, which has hitherto 

 been subspherical, takes the form of an egg, the smaller end 

 turned upwards, the broader resting on the bottom. The shell 

 cracks and its upper half comes away in pieces, allowing the 

 embryo to push the smaller end of its body out into the water. 

 The embryo is now more transparent than heretofore ; the 

 ectoderm and endoderm are well defined, and the mesogloea 

 is established as a boundary between them. A clear space, 

 the beginning of the gastrovascular cavity, makes its appear- 

 ance in the endoderm of the emergent upper half of the body, 

 and this space gradually extends, probably as a result of the 

 disintegration of the central cells of the endodermic mass, to the 

 lower end. As the embryo protrudes itself further from the 

 shell its upper end expands and the tentacles are formed as a 

 ring of small hollow outgrowths. The mouth is formed as a split 

 in the centre of the ring of tentacles, placing the gastrovascular 

 cavity in communication with the exterior. The young Hydra 

 is now complete. It creeps out of the half of the shell in 

 which it has up till now been seated, attaches itself to some 

 other object and enters upon its adult career. 



