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 (ji/ws, Fig. 466). 

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



tc 



him 



FIG. 459. Suction through the segmenting of I'lirosiima r/ii/dnicum at the stage of 

 thirty-six lilastomeres. (After Juliii.) 



lit HI, blastoincre ; ./; follicle cells ; t.<; test cells some have wandered in unionist, (lie 

 lilastonieres ; ;/, unsegmented yolk. 



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

 They are surrounded by islands of cytoplasm (y.c, Fig. 460). 



Between the blastoderm and the yolk a cavity appears which is 

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

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



Ic 



FIG. 460. Longitudinal sections through two embryos of Pyrosom '////"///</'///, in order 



to show the formation of organs in the Cyathozooid. (Alter Julin.) 



A, seetion tlinmgli younger embryo. 13, section tlmmxli older embryo. I'm/, endodei-ni t'oi'niiii.u m.,1 

 of archenteron and beginning to extend on to its floor ; /, follicle \\itli nuclei ; ./>. nei ve plate formed 

 by an ectodermic thickening which becomes invaginatc.l ; per, pericardia! tniie; /.//. |ihaiyn\; t.<; test 

 cells ; H.C, yolk i ells. 



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

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

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

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

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



