294 STUDY OF YOUNG CHICK EMBRYOS. 



and, third, the somewhat thicker cell-layer in the embryo proper. For an ac- 

 count of the cells of the area opaca and area pellucida see page 86. The ento- 

 derm in the embryo presents considerable variations in thickness which have 

 been pointed out in the descriptions of the sections. Where it is thick enough 

 to permit it, the nuclei are disposed in several layers, and in such places we find 

 that the nuclear divisions take place only in the superficial portion of the ento- 

 derm, the phenomenon here being similar to that which we have already noted in 

 the ectoderm. The notochord has a sharply defined outline, as if bounded by 

 a distinct membrane. It contains nuclei which are quite closely placed, but it 

 does not show, at least in ordinary preparations, any recognizable division into 

 separate cells. 



The mesoderm offers several varieties, not so much in the character of the 

 single cells as in their methods of grouping. We notice, first, that there are 

 parts of the mesoderm which are quite thick, and in which we cannot perceive 

 any division into mesothelium and mesenchyma. Such a thick layer of mesoderm 

 may be observed at either side of the pharynx (Figs. 158, Ph, and 159), or, again, 

 toward the caudal end of the embryo in both the somatopleure and splanchno- 

 pleure, occupying a larger territory in the former than in the latter (Fig. 163). 

 But for the most part the mesoderm has progressed beyond this stage and shows 

 clearly the differentiation of a thin mesothelial layer lining the coelom and the 

 scattered mesenchymal cells. The mesothelium is quite thin in some parts, 

 almost or quite as thin as adult endothelium. The mesenchyma consists of cells 

 with small protoplasmic bodies connected together by fine threads of protoplasm 

 and with a transparent homogeneous matrix between the cells. It varies greatly 

 in appearance according as the cells are more or less closely crowded together, or 

 widely separated from one another. These differences we designate as varying 

 degrees of condensation in the mesenchyma. The variations occur in a per- 

 fectly definite and constant manner, though we are far from understanding yet 

 either the cause or the morphological significance of these variations. The sec- 

 ondary segments vary greatly in structure, because they are in unlike stages of 

 differentiation, those toward the tail being least, and those in the cervical region 

 most advanced. We can, therefore, in a single embryo observe several phases 

 of the breaking-up of the inner wall of the segment to form mesenchyma about 

 the medullary tube and notochord. The transformation is accomplished by a 

 spreading out and moving asunder of the cells, and we can also trace a gradual 

 differentiation of the muscle plate, out of the inner portion of the segment. The 

 external layer, or so-called cutis plate, offers an apparently more or less epithe- 

 lioid structure in all of the segments. The Wolffian duct is differentiated only 

 through a part of the embryo. It is a small cord of cells that has as yet no cen- 



