130 A HUMAN EMBRYO OF TWENTY-FOUR PAIRS OF SOMITES. 



bend is very gradual, and the head itself much narrower and more pointed than 

 in either Janosik's specimen or mine. Although somewhat, similar in appearance 

 to His's 18 embryos "Lg," "Sch," and "BB," my specimen differs from them 

 in regard to the position of the ventral flexure of the back. In His's specimens 

 the much-discussed bend in the back is always placed opposite the attachment 

 of the yolk sac and stalk. In my specimen, however, the bend is placed further 

 caudally, and the portion of the body which is bent backward is relatively shorter. 

 The twin specimens which Watt 48 describes show definite ventral curvatures of 

 the back, but these also are placed relatively higher up and are not as sharp as 

 the bend in my specimen. 



INTEGUMENT. 



In general, it may be said that the integument of the embryo is made up of 

 one or two layers of ectodermal cells. The thickness of this layer and the shape 

 and size of its cells, however, vary considerably in different regions of the body. 



Over the sides of the head the ectoderm is thin, being composed for the most 

 part of two layers of flattened cells with rounded nuclei. On the dorsum and 

 front of the head the epithelium is still thinner, there being but one layer of 

 flattened cells. Over the optic vesicles, where the lens placodes will later develop, 

 there is as yet no indication of thickening, but in the region of the gill-arches the 

 epithelium is considerable thicker, its cells being either cubical or columnar in shape. 



In the region of the hindbrain there is seen from the surface a minute aper- 

 ture (plate 2, fig. 1). Here the integument dips in and expands to form a sac, 

 the auditory vesicle. This is flattened laterally, and approximately triangular 

 in external view. It is closely applied to the brain, overlying the fifth and a part 

 of the sixth neuromeres. With the exception of its form it is quite closely in 

 accord with the more spherical vesicle of Thompson's embryo. The walls of the 

 auditory vesicle are much thicker than the overlying ectoderm, and exhibit two 

 or three layers of rounded or oval nuclei. Mitotic figures in the auditory vesicle 

 are numerous. 



The integument in the region of the mouth shows no especial thickenings. 

 A few clusters of cells (the remains of the oral plate) are attached to it; anteriorly, 

 one such cluster is found at about the level of the cephalic end of the notochord; 

 other clusters are found on the sides and ventral wall of the oral cavity. The 

 epithelium of the roof of the mouth is placed in close apposition to the floor of the 

 forebrain, being separated from it by a few strands of mesenchyma only. There 

 is no doubt that this portion of the oral integument is destined to become the 

 anterior lobe of the hypophysis, but as yet there is no definite differentiation of 

 this organ. 



The integument of the body-wall of the embryo overlying the pericardial 

 cavity is very thin, suggesting a stretching-out of the epithelium. It is also thin 

 over the dorsum of the trunk and over the mesodermic somites. The integument 

 which overlies the body-wall in the region of the umbilical vein is somewhat 

 thicker and its nuclei are more closely packed together. It gradually thins out 

 again as it is reflected to form the amnion. 



