RABBIT VESICLES IN ALCOHOL. 309 



the nuclei of the two layers are seen to alternate more or less with one another. 

 This characteristic disposition is not kept everywhere, but is subject to consider- 

 able variations. In the very most advanced ova of six days a small spot some- 

 times can be observed in the embryonic shield which is noticeable from its 

 greater opacity. This spot corresponds to Hensen's knot, but it does not usually 

 show itself distinctly until considerably later. 



Vesicles at Seven Days. — Vesicles at this age vary greatly in size, and the 

 stage of development varies with the size — how exactly, we do not yet know. 

 Preliminarily we may fix on the normal size as being that of vesicles, the greatest 

 diameter 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. 

 175, 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 yig. 175.— Blastodermic 

 end is near the center of the shield, its rounded end a little Vesicle of a Rabbit 



distance behind the point of the shield. The darker area ^^ Seven Days. 



owes its formation to the appearance of a new layer of ^^' Embryonic shield, mes, 



^^- . ^ Mesoderm. (Semi-dia- 



cells between the ectoderm and entoderm. This layer grammatic.) 



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 of the primitive streak and notochordal canal, and perhaps do 

 not belong to the mesoderm. In the region outside the embryonic shield the outer 

 layer is easily distinguished; its cells have marked outlines, but are of smaller 

 dimensions than in earlier stages, their nuclei are large, for the most part oval, 

 and contain several highly refringent and conspicuous 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 epi- 

 thelial arrangement, although at the edge of the expanding layer the cells are still 

 more or less scattered. The entodermal cells are easily distinguished by chang- 

 ing the focus of the microscope, when their darker protoplasm and smaller size. 



