XVII 



PEOTOCHOEDATA 



633 



of the development of this form, many of the test cells survive and 

 give rise to the phosphorescent organs of the adult {flws, Fig. 466). 

 In the yolk there are nuclei which are derived by division from 



Fig. 459. — Section through the segmenting of Pyrosoma gir/antenm at the stage of 



thirty-six blastomeres. (After Julin.) 



blvi, blastomere ; /, follicle cells ; t.c, test cells — some have wandered in amongst the 



blastomeres ; y, unsegmented yolk. 



the nuclei of the blastomeres, and which we may term yolk cells. 

 They are surrounded by islands of cytoplasm {y.G, Fig. 460). 



Between the blastoderm and the yolk a caAdty appears which is 

 the archenteric cavity, and at this stage, according to Juhn, the 

 blastoderm consists, throughout the greater part of its extent, of 



Fig. 460. — Longitudinal sections through two embryos of Pyrosoma gigantrum, in order 

 to show the formation of organs in the Cyathozooid. (After Jnlin.) 



A, section through younger embryo. B, section through older embryo, ewrf, endoderm forming roof 

 of archenteron and beginning to extend on to its floor ; /, follicle with nuclei ; n.-p, nerve plate formed 

 by an ectodermic thickening which becomes invaginated ; jser, pericardial tube ; pli^ pharynx ; t.c, test 

 cells ; y.Cj yolk cells. 



two layers. The outer of these layers constitutes the ectoderm, 

 and the inner, which forms the immediate roof of the archenteric 

 cavity, is endoderm. The two layers pass into each other along a 

 definite lip which Julin identifies with the dorsal and anterior lip of the 

 blastopore, corresponding to the region x in the gastrula of Amphioxus. 



