HISTOLOGICAL DIFFERENTIATION. 293 



Histological Differentiation of the Chick Embryo with Three Gill Clefts. 



It is important that the student make a thorough examination and study 

 with a high power of all the cells and tissues of the embryo at this stage so as to 

 familiarize himself with the embryonic characteristics of the germ-layers. The 

 cellular homogeneity of the embryo is strikingly evidenced by the nuclei, which 

 in all parts of the embryo are very similar in size, shape, and structure. They 

 are all rounded in form, varying between spherical and slightly oval outlines, 

 which are seldom quite regular. The outline of the nucleus is always well 

 marked, there being a superficial layer of nuclear substance, which gives a darker 

 appearance to the edge of the nucleus. In the interior there is a single or some- 

 times two, very rarely three, nucleoli, which are quite large and stain deeply. 

 The strands of substance between the nucleolus and the outer part of the nucleus 

 are very slight, and the space around the nucleolus, therefore, appears light. 

 The protoplasm of the cells is never large in amount, so that the cell-body about 

 each nucleus is not conspicuous, except in the case of the blood-corpuscles, which 

 are, in this respect, somewhat more advanced than the other cells of the embryo. 



The ectoderm offers chiefly variations in its thickness, being very much at- 

 tenuated in some parts, as, for instance, in the posterior portion of the head, where 

 the outer ectoderm overlies the hind-brain. Most of the epidermal parts have 

 begun to increase in thickness, and contain nuclei in two or even three layers. 

 There are several special thickenings of the epidermal layer, for which the name 

 of plakodes has been proposed. At the present stage three pairs of plakodes are 

 seen. The first is the pair of areas which are to be invaginated to form the 

 olfactory pits; the second is the pair which are already invaginated to form 

 the anlages of the lenses of the eyes, and the third pair are also invaginated to 

 form the otocysts. The portion of the ectoderm which forms the medullary tube 

 is also very much thickened, except, of course, so far as the floor-plate and deck- 

 plate have been differentiated. In both the plakodes and in the thickened por- 

 tions of the medullary wall the nuclei occupy nearly the whole thickness of the 

 layer, being themselves several layers deep. They are, however, partially 

 absent from that portion of the ectoderm which is near the original external or 

 free surface. Close to this surface there are, however, a certain number of 

 nuclei, the large majority of which are in various phases of division, as shown by 

 the numerous mitotic figures. No mitoses appear, except in the superficial por- 

 tion of the layer. Over the greater part of the amnion the ectoderm is so very 

 thin as to resemble almost an adult endothelium, but over the chorion or serous 

 membrane it is a little thicker. 



The entoderm appears in three distinct forms: first, the large, long, 

 columnar cells of the area opaca ; second, the very thin cells of the area pellucida ; 



