II 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 
Fie. 25.—Embryology of Ascidian. A, mature ovum: oli, follicle-cell ; mm, membrane ; 
mn, nucleus; p, protoplasm ; ¢.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 ; K, transverse section of tail of larva. ar, Archenteron ; 
at, atrial invagination ; av, otocyst ; b.c, blastocoel ; b.p, blastopore ; ch, notochord ; 
ep, epiblast ; /, tail-fin ; hy, hypoblast ; m.b, mesoblast ; mes, mesenteron ; musc, 
muscle-cell ; 7.c, neural canal; ne.c, neurenteric canal; 7.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, ch). 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 
