56 SEGMENTATION. 



The segmentation, which is preceded by active movements of the 

 germinal disc, is meroblastic. There is nothing very special to note 

 with reference to its general features, but while in large ova like 

 those of the Salmon the first furrows only penetrate for a certain 

 depth through the germinal disc, in small ova like those of the 

 Herring, they extend through the whole thickness of the disc. 

 During the segmentation a great increase in the bulk of the blas- 

 toderm takes place. 



In hardened specimens a small cavity amongst the segmentation 

 spheres may be present at any early stage; but it is probably an arti- 

 ficial product, and in any case has nothing to do with the true 

 segmentation cavity, which does not appear till near the close of 

 segmentation. The peripheral layer of granular matter, continuous 

 with the germinal disc, does not undergo division, but it becomes 

 during the segmentation specially thickened and then spreads itself 

 under the edge of the blastoderm ; and, while remaining thicker in 

 this region, gradually grows inwards so as to form a continuous sub- 

 blastodermic layer. In this layer nuclei appear, which are equivalent 

 to those in the Elasmobranch ovum. A considerable number of 

 these nuclei often become visible simultaneously (van Beneden, 

 No. 60) and they are usually believed to arise spontaneously, though 

 this is still doubtful \ Around these nuclei portions of protoplasm 

 are segmented off, and cells are thus formed, which enter the blasto- 

 derm, and have nearly the same destination as the homologous cells 

 of the Elasmobranch ovum. 



During the later stages of segmentation one end of the blastoderm 

 becomes thickened and forms the embryonic swelling ; 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 sn])posed that this cavity was equivalent to the 

 segmentation cavity of Elasmobranchs in its earliest condition, but Bam- 

 beke 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 Elasmobi-anchii, 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 (1) 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 



1 Vide Vol. I. p. 89. 



