HISTOLOGICAL DIFFERENTIATION. 217 



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 supefficial layer of nuclear substance, which gives a darker appear- 

 ance to the edge of the nucleus. In the interior there is a single or sometimes 

 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 be- 

 gun 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 (compare page 76). At the present stage three 

 pairs of plakodes are seen. The first is the pair of areas which are to be invagi- 

 nated to form the olfactory pits; the second is the pair which are already invagi- 

 nated to form the anlages of the lenses of the eyes, and the third pair is 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 portions of the medullary wall the nuclei occupy nearly the whole thick- 

 ness of the layer, being themselves several layers deep. They are, however, par- 

 tially 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 tne superficial portion 

 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 mem- 

 brane 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; and, 

 third, the somewhat thicker cell-layer in the embryo proper. For an account of 

 the cells of the area opaca and area pellucida see page 64. The entoderm 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 entoderm, the phenome- 

 non here being similar to that which we have already noted in the ectoderm. The 



