70 SEGMENTATION. 



During the later stages of segmentation one end of the 

 blastoderm becomes thickened and forms the embryonic swell- 

 ing ; and a cavity appears between the blastoderm and the yolk 

 which is excentrically situated near the non-embryonic part of 

 the blastoderm. This cavity is the true segmentation cavity. 

 Both the cavity and the embryonic swelling are seen in section 

 in fig. 31 A and B. 



In Leuciscus rutilus Bambeke describes a cavity as appearing in the 

 middle of the blastoderm during the later stages of segmentation. From his 

 figures it might be supposed that this cavity was equivalent to the segment- 

 ation cavity of Elasmobranchs in its earliest condition, but Bambeke states 

 that it disappears and that it has no connection with the true segmentation 

 cavity. Bambeke and other investigators have failed to recognize the 

 homology of the segmentation cavity in Teleostei with that in Elasmo- 

 branchii, Amphibia, etc. 



With the appearance of the segmentation cavity the portion 

 of the blastoderm which forms its roof becomes thinned out, so 

 that the whole blastoderm consists of (i) a thickened edge 

 especially prominent at one point where it forms the embryonic 

 swelling, and (2) a thinner central portion. The changes which 

 now take place result in the differentiation of the embryonic 

 layers, and in the rapid extension of the blastoderm round the 

 yolk, accompanied by a diminution in its thickness. 



A 



FIG. 31. LONGITUDINAL SECTIONS THROUGH THE BLASTODERM OF THE 



TROUT AT AN EARLY STAGE OF DEVELOPMENT. 



A. at the close of the segmentation; B. after the differentiation of the germinal layers. 

 ep' . epidermic layer of the epiblast; sc, segmentation cavity. 



The first differentiation of the layers consists in a single row 

 of cells on the surface of the blastoderm becoming distinctly 



