186 THE BLASTOPORE. 



the medullar} 7 groove is formed, the mesoblast becomes divided, exactly 

 as in Lacerta and Elasmobranchii, into two independent lateral plates, 

 which are not continuous across the middle line (fig. 142 me). The 

 hypoblast cells are flattened laterally, but become columnar beneath 

 the medullary plate (fig. 142). 



In tracing the changes which take place in the relations of the 

 layers, in passing from the region of the embryo to that of the 

 primitive streak, it will be convenient to follow the account given by 

 Schafer for the guinea-pig (No. 190), which on this point is far fuller 

 and more satisfactory than that of other observers. In doing so I 

 shall leave out of consideration the fact (fully dealt with later in this 

 chapter) that the layers in the guinea-pig are inverted. Fig. 143 

 represents a series of sections through this part in the guinea-pig. 

 The anterior section (D) passes through the medullary groove near 

 its hinder end. The commencement of the primitive streak is 

 marked by a slight prominence on the floor of the medullary groove 

 between the two diverging medullary folds (fig. 143 C, ae). Where 

 this prominence becomes first apparent the epiblast and hypoblast 



FIG. 142. TRANSVERSE SECTION THROUGH AN EMBRYO RABBIT OF EIGHT DAYS. 

 cp. epiblast; me. mesoblast; hi/, hypoblast; ///. medullary groove. 



are united together. The mesoblast plates at the two sides remain 

 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, f), which in the section next 

 following becomes detached from the hypoblast and forms a solid 

 keel projecting from the epiblast. In the following section the hitherto 

 independent mesoblast plates become united with this keel (fig. 

 143 A) ; and in the posterior sections, through the part of the 

 primitive streak with the primitive groove, the epiblast and mesoblast 

 continue to be united in the axial line, but the hypoblast remains 

 distinct. These peculiar relations may shortly be described by saying 

 that in the axial line the hypoblast becomes united with the epiblast at 

 the posterior end of the embryo ; and that the cells which connect the 

 hypoblast and epiblast are posteriorly continuous with the fused epi- 

 blast and mesoblast of the primitive streak, the hypoblast in the region 

 of the primitive streak having become distinct from the 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 i*abbit, which is in 

 many respects very instructive. In this stage the thickened axial 

 portion of the hypoblast in the region of the embryo becomes sepa- 



