234 



ZOOLOGY. 



and folds, the myoccelomic pouches, and the notochord, were 

 formed simultaneously. The corresponding stage is found 

 in the frog, allowing for the absence of actual myocoelomic 

 pouches. 



The neural plate resembles that of Amphioxus, except 

 that from the first it is broader in front (the future brain) 

 than behind. Moreover, as the neural folds grow up, the 

 edges of the plate rise with them, so that the plate becomes a 

 neural tube as soon as the folds close in. (See figs. 120 and 

 121, and contrast with figs. 113 and 115.) The enlarged 

 front end of the neural tube becomes very early constricted 

 into three cerebral vesicles. The hind ends of the folds do 

 not enclose the blastopore for some time, but do so at last, 

 forming a neurenteric canal as in Amphioxus. 



The notochord is formed from the hypoblast of the dorsal 

 middle line, just as in Amphioxus ; bub it never extends in 

 front of the middle of the second cerebral vesicle (fig. 121.) 



9. Divisions of the Mesoblast. 



Fig. 120. TRANSVERSE SECTION OF EMBRYO OF FROO. 

 (After Marshall.) 



The mesoblast, at first 

 solid, soon splits 

 on each side 

 into two layers 

 (fig. 120), with 

 a myoccelomic 

 cavity between 

 them. In Am- 

 phioxus we saw 

 that a paired 

 metameric 

 series of such 

 cavities was 

 present from 

 the first ap- 

 pearance of the 

 mesoblast; that 

 each pouch be- 

 came after- 

 wards divided 



into dorsal and ventral portions: that the dorsal portions 

 retained their metamerism but lost their cavities, becoming 



