170 STUDY OF THE SEGMENTATION OF THE OVUM. 



of development varies with the size how exactly, we do not yet know. Prelimi- 

 narily we may fix on the normal size as being that of vesicles the greatest diam- 

 eter of which is about 4 mm. Such vesicles are somewhat oval in shape and slightly 

 flattened on the side bearing the embryonic shield. The membrane enclosing 

 them is very thin; the albuminoid layer can scarcely be distinguished, but the zona 

 pellucida is very distinct. The shield (Fig. 125, Sh) is. somewhat elongated and 

 distinctly pear-shaped. Its long axis is parallel with that of the vesicle. It varies 

 greatly in its dimensions. Shields i mm. wide and from 1.3 to 1.4 mm. long are 

 not uncommon. The student will be likely to encounter other dimensions. The 

 most striking addition is the appearance of a darker area, mes, at the posterior or 

 pointed end of the shield. This darker area is also somewhat pear-shaped, but 

 its pointed end is near the center of the shield, its rounded 

 end a little distance behind the point of the shield. The 

 darker area owes its formation to the appearance of a new 

 layer of cells between the ectoderm and entoderm. This 

 layer consists of loosely connected cells with rounded nuclei 

 easily distinguishable in surface views from those of the 

 subzonal layer. The greater part of these cells are certainly 

 mesodermic, but a portion of them share in the formation 

 FIG i2<c BLASTODERMIC ^ * ne primitive streak and notochordal canal, and perhaps 

 VESICLE OF A RABBIT do not belong to the mesoderm. In the region outside the 

 AT SEVEN DAYS. embryonic shield the outer layer is easily distinguished; its 



Sh, Embryonic shield, mes, 111 i j ^T r j- 



/c . ,. cells have marked outlines, but are of smaller dimensions 



Mesoderm. (Semi-dia- 



grammatic.) x 6 diams. than in earlier stages, their nuclei are large, for the most 

 part oval, and contain several highly refringent and con- 

 spicuous granules. The number of granules varies; when there are only two or 

 three, they are apt to be elongated as if several small granules had united. The 

 entodermal cells have spread well past the equator of the vesicle and present, 

 for the most part, a distinctly epithelial arrangement, although at the edge of the 

 expanding layer the cells are still more or less scattered. The entodermal cells 

 are easily distinguished by changing the focus of the microscope, when their darker 

 protoplasm and smaller size, together with their smaller darker nuclei, make them 

 readily recognizable. The granules in the entodermic nuclei are smaller and more 

 numerous than in the overlying ectodermal nuclei. 



During the next few hours further changes ensue. At the apex of the pear- 

 shaped mesodermic area there appears a small spot, which is known as Hensen's 

 knot. At first Hensen's knot consists of a little thickening accompanied by a union 

 of the cells of the middle layer with those of the overlying ectoderm. Next occurs 

 the development of the primitive streak, which runs from Hensen's knot backward 

 toward the apex of the embryonic shield, and very soon thereafter along the line 

 of the primitive streak there develops the external and shallow primitive groove. 

 At Hensen's knot the three layers now are found to be t intimately united, so that, 



