38 THE E PI BLAST. 



sented in white and is seen to be continuous at the lip of the blasto- 

 pore {x) with the shaded hypoblast. Between the epiblast and hypo- 

 blast is seen one of the lateral plates of mesoblast, represented by 

 black cells with clear outlines. The non-embryonic lower layer cells of 

 the blastoderm are represented in the same manner as the mesoblast 

 of the body. The alimentary cavity is shewn at al, and below it is 

 seen the yolk with nuclei (n). The segmentation cavity is re- 

 presented as still persisting, though by this stage it would have 

 disappeared. 



As to the growth of the blastoderm it may be noted that it has 

 greatly extended itself over the yolk. Its edge in the meantime 



J^P 



Fio. 20. Three sections through a Pristiurus embryo somewhat younger 



THAN FIG. 28 C. 



A. Section through the cephalic plate. 



B. Section through the posterior part of the cephalic plate. 



C. Section through the trunk. 



ch. notochord; mg. medullary groove; al. alimentary tract; Ip. lateral plate of 

 mesoblast; i^p. body cavity. 



forms a marked ridge, which is due not so much to a thickening as to an 

 arching of the epiblast. This ridge is continuous with the embryonic 

 rim, which gradually concentrates itself into two prominences, one on 

 each side of the tail of the embryo, mainly formed of masses of 

 undifferentiated lower layer cells. These prominences will be called 

 the caudal swellings. 



By this stage the three layers of the body, the epiblast, mesoblast, 

 and hypoblast, have become definitely established. The further history 

 of these layers may now be briefly traced. 



Epiblast. While the greater part of the epiblast becomes con- 

 verted into the external epidermis, from which involutions give rise 

 to the olfactory and auditory pits, the lens of the eye, the mouth 

 cavity, and anus, the part of it lining the medullary groove becomes 

 converted into the central nervous system and optic cup. The medullary 



