46 DEAN. [Vol. XI. 



"aux d^pens des cellules de I'entoderme, qui est tout au moins 

 le principal facteur dans sa formation, et non pas aux depens du 

 bourrelet marginal." Of the entoderm cells at the margin of 

 the ingrowing coelcnteron the more superficial are in their turn 

 continually becoming mesodermic ; " apres s'etre multipliees, 

 elles prennent des caracteres tr^s semblables a ceux des cellules 

 du bourrelet marginal, et de cette fagon, tant que la cavite 

 digestive primitive s'etend, le mesoderme s'accroit de bas en 

 haut aux depens de I'entoderme." From sections of well pre- 

 served material a quite different interpretation of the origin 

 and growth of the mesoblast is to be obtained. In PI. IV, 

 Fig. 6'^, it will be seen that in a very early stage of gastrula- 

 tion the lip of the blastopore is already divided into its three 

 germ layers ; the outer layer thickens and is deeply inflected at 

 the lip of the blastopore, to a point where it becomes connected 

 with the middle and inner layers : here the inner layer is thin- 

 nest, thence it widens, but near the end of the coelenteron, 

 merging with the yolk-laden cells, its boundary is no longer to 

 be traced : it is now indistinguishable from the middle layer 

 which from the lip of the blastopore up to this point has 

 remained distinct. At a later stage, Fig. 64, the inner 

 layer of both dorsal and ventral lips is seen to be continuous 

 with the large entoderm cells of the yolk mass : at the Rand- 

 wulst the three layers merge ; in the region ectad of the end 

 of the coelenteron the mesoderm passes into the yolk mass. 

 In Figs. 65, 66, the germ layers are seen more widely 

 separated, and their confluence in the Randwulst is more 

 prominent. In the section of the greatly reduced blastopore, 

 Fig. 6^, the middle layer in this region is to be clearly 

 seen separate from the inner layer and confluent with the yolk 

 cell mass. In transverse sections of this stage, F'ig. 70, 

 the presence of gastral mesoderm may be established for a 

 short distance in front of the blastopore (equivalent to about 

 one-sixth of the length of the embryo of PI. Ill, Fig. 51); 

 further forward than this the layer of mesoblast decreases not- 

 ably in thickness, but always maintains its connection laterally 

 with the yolk mass. 



The mode of early development of the middle germ layer of 



