32 



Mediterranean, and from the end of May to the end of 

 June at Trieste. 



Segmentation of the egg results in the formation of 

 a spherical embryo, which consists of a single layer of 

 cells enclosing a cavity filled with gelatinous matter. 

 This is a coeloblastula. Immediately after the completion 

 of this initial stage, a depression, due to invagination of 

 the wall of the cceloblastnla, appears at one pole, and the 

 invaginating portion soon becomes bilaminar. The slit- 

 like aperture of the invagination is the blastopore. The 

 embryo is now a gastrula. The cells of the inner layer 

 of the invagination migrate into the gelatinous matter to 

 form mesenchyme (PI. YI., fig. 63, mes.). The external 

 surface of the embryo is covered with cilia, the action of 

 which causes it to rotate within the egg membrane. 

 Presently the blastopore closes completely, and the 

 archenteron becomes a closed vesicle. A circular furrow 

 now appears in its walls and, gradually deepening, 

 eventually separates the archenteric cavity into two 

 distinct vesicles, of which the anterior is slightly the 

 larger (fig. 63). The latter gives rise to the intestine 

 and the hydrocoel, while from the posterior vesicle the 

 coelom and the chambered organ are derived. The 

 posterior vesicle elongates transversely ; and concurrently, 

 the anterior one is produced into horn-like dorsal and 

 ventral extensions (fig. 64, me. hy. V5.), which grow around 

 it until they touch, but do not communicate. The 

 two ends of the posterior vesicle now enlarge, while the 

 median portion, encircled by the horns of the anterior 

 vesicle, becomes more tubular. The embryo is now 

 bilaterally symmetrical, and begins to assume an oval 

 form. A ventral outgrowth from the anterior vesicle now 

 forms the rudiniient of the hydrocoel {ru. hy.), and the 

 vesicle itself becomes the intestine. The enlarged ends of 



