218 STUDY OF YOUNG CHICK EMBRYOS. 



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. 156, Ph, and 157), or, again, 

 toward the caudal end of the embryo in both the somatopleure and splanchnopleure, 

 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 proto- 

 plasmic bodies connected together by fine threads of protoplasm and with a trans- 

 parent homogeneous matrix between the cells. It varies greatly in appearance ac- 

 cording as the cells are more or less closely crowded together, or widely separated 

 from one another. These differences we designate as varying degrees of condensa- 

 tion in the mesenchyma. The variations occur in a perfectly definite and constant 

 manner, though we are far from understanding yet either the cause or the morpho- 

 logical significance of these variations. The secondary somites 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 somite 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 somite. The external layer, or so-called cutis-plate, 

 offers an apparently more ,or less epithelioid structure in all of the somites. The 

 Wolffian duct is differentiated only through a part of the embryo. It is a small 

 cord of cells that has as yet no central cavity. The blood-vessels are formed solely 

 by the endothelium (angioblast). There is nowhere any condensation of the mesen- 

 chyma about the blood-vessels as yet. There are no capillaries whatever in the 

 embryo. One of the most important vascular modifications has, however, been 

 initiated in the anlage of the liver, where we find the vascular endothelium com- 

 ing into close contact with the entodermal cells of the liver, preparatory to the 

 later complete differentiation of the hepatic sinusoids. The blood-corpuscles are 

 round in form with fairly distinct outlines. Their protoplasmic bodies are much 

 larger than those of any other cells of the embryo at this stage, but their nuclei 

 resemble in size and structure those of other tissues. 



