MAMMALIA. 



225 





in the meantime quite free. Slightly further back, but before 

 the primitive groove is reached, the epiblast and hypoblast are 

 connected together by a cord of cells (fig. 143 B, /), which in 

 the section next following becomes detached from the hypoblast 

 and forms a solid keel pro- 

 jecting from the epiblast. 

 In the following section the 

 hitherto independent meso- 

 blast plates become united 

 with this keel (fig. 143 A); 

 and in the posterior sec- 

 tions, through the part of 

 the primitive streak with 

 the primitive groove, the 

 epiblast and mesoblast con- 

 tinue to be united in the 

 axial line, but the hypoblast 

 remains distinct. These pe- 

 culiar relations may shortly 

 be described by saying that 

 in the axial line the hypo- 

 blast becomes united witJi 

 the epiblast at the posterior 

 end of the embryo ; and 

 that the cells which con- 

 nect the hypoblast and epi- 

 blast are posteriorly con- 

 tinuous with the fused epi- 

 blast and mesoblast of the 

 primitive streak, the hypo- 

 blast in the region of the 

 primitive streak having be- 



the 



FlG. 143. A SERIES OF TRANSVERSE SEC- 

 TIONS THROUGH THE JUNCTION OF THE 

 PRIMITIVE STREAK AND MEDULLARY GROOVE 



OF A YOUNG GUINEA-PIG. (After Schafer.) 

 A. is the posterior section. 



e. epiblast; in. mesoblast; h. hypoblast; 

 ae. axial epiblast of the primitive streak ; 

 ah. axial hypoblast attached in B. and C. to 

 the epiblast at the rudimentary blastopore ; 

 ng. medullary groove ; f. rudimentary blas- 

 topore. 



come distinct from 

 other layers. 



The peculiar relations just described, which hold also for the 

 rabbit, receive their full explanation by a comparison of the 

 Mammal with the Bird and the Lizard, but before entering into 

 this comparison, it will be well to describe the next stage in the 

 rabbit, which is in many respects very instructive. In this stage 



B III. 15 



