EMBRYONIC DEVELOPMENT 



57 



flattened, and the blastopore indicates its posterior end. Around 

 the blastopore the large ectoderm cells form a medullary plate, 

 along which a groove (the medullary groove), runs forwards, 

 bounded at the sides by medullary folds which meet behind the 

 blastopore. Underneath the posterior part of the medullary groove 



FIG. 25. Embryology of Ascidian. A, mature ovum : foil, follicle-cell ; m, membrane ; 

 n, nucleus ; p, protoplasm ; t.c, test-cell ; B, mature spermatozoon ; C, segmentation- 

 stage in section to show blastocoel ; D, early gastrula-stage ; E, later gastrula- stage ; 

 F, later embryo showing rudiments of notochord and neural tube ; G, transverse 

 section of body of embryo showing mesoblast and formation of neural canal ; H. late 

 embryo showing body and tail, notochord, neural canal, and mesenteron ; I, young 

 larva ready to be hatched ; TL, transverse section of tail of larva, ar, Archenteron ; 

 at, atrial invagination ; au, otocyst ; b.c, blastocoel ; b.p, blastopore ; ch, notochord ; 

 ep, epiblast ; f, tail-fin ; hy, hypoblast ; m.b, mesoblast ; mes, mesenteron ; TWMSC, 

 muscle-cell : n.c, neural canal ; ne.c, neurenteric canal ; n.v, neural vesicle ; oc, 

 ocellus. (Modified from Kowalevsky and others.) 



certain of the hypoblast cells from the dorsal wall of the archen- 

 teron, in the median line, form a band extending forwards (Fig. 

 25, E, c/t). This band separates off from the hypoblast, which 

 closes in beneath it, and thus gives rise to the notochord (Fig. 

 25, F). The more lateral and posterior cells become mesoblast, 

 and separate off as lateral plates, which show no trace of metameric 

 segmentation (Fig. 25, G). The remainder of the archenteron 



