4 6 



FORMATION OF MESOBLAST. 



tudinal section through the embryo (fig. 19 B). In this figure 

 the epiblast is represented in white and is seen to be continuous 

 at the lip of the blastopore (x) with the shaded hypoblast. 

 Between the epiblast and hypoblast is seen one of the lateral 

 plates of mesoblast, represented by black cells with clear out- 

 lines. 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 (ii). The segmentation cavity is re- 

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

 disappeared. 



FIG. 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 ; lp. lateral plate of 

 mesoblast ; //. body cavity. 



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 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 con- 

 centrates 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. 



