]2 YOUNG TWIN HUMAN EMBRYOS WITH 17-19 PAIRED SOMITES. 



The above description shows this reflection of the amnion in this embryo to be essen- 

 tially similar to that given by Low for embryo Pfannenstiel III. The embryo described by 

 Thompson (1907) shows the nearest stage described in advance of this. Here the whole 

 heart region projects into the cavity, the amnion being reflected from the ventral body wall 

 on the line of the septum transversum. The amnion is also shown beginning to pass out 

 over the yolk stalk, so that the large aperture of the umbilical vesicle of previous stages 

 here begins to be constricted. Posteriorly the amnion only covers the narrow, dorsal 

 aspect of the belly stalk, and still exhibits the narrow, conical prolongation of the cavity. 



There are no differences to be noted regarding the amnion of Embryo V. 



THE CHORION. 



The chorion agrees in every respect with the description given of other embryos. It 

 is from 0.09 to 0.18 mm. thick and the villi arising from it are 0.17 to 0.25 mm. in diameter 

 and 1.0 mm. to 1.6 mm. in length. These measurements are not very much greater than 

 those given by Dandy (1910) for an embryo about 2 mm. long, with only 7 pairs of somites. 

 As most of the chorion had been removed before the embryo was received, it can not be 

 stated whether the whole sac was equally covered by villi, or not, 



The inner surface of the chorion and the interior of the villi are formed of loose mesen- 

 chyme consisting of branching spindle and stellate cells. The nuclei of the cells are oval 

 and vesicular, the protoplasm stains very lightly and forms only a thin covering over the 

 nucleus and the numerous fine branching processes. The intercellular tissue spaces are 

 large, and contain a clear jelly-like substance. Numerous large blood-vessels are found 

 everywhere in this tissue in the chorion and its villi. 



The outer surface of the chorion and villi is formed of two epithelial layers of cells. 

 The inner layer, the Langhans cells, lying next the mesenchyme, have moderately staining 

 nuclei and protoplasm. In sections vertical to the plane of the cell layer, the cells appear 

 cubical in form and are very little deeper than is just sufficient to contain the large round 

 vesicular nuclei. Cell boundaries are evident and distinct, In sections parallel to the 

 surface of this layer the cells appear large and polygonal in outline, the majority being 5- 

 sided. The outer layer is a syncytium formed of a densely stained, granular protoplasm 

 containing deeply stained, flattened, oval nuclei which lie with their long axis parallel to the 

 surface of the layer. This layer is thinner than the Langhans layer and contains fewer 

 nuclei, and I find, contrary to Dandy (1910), that the nuclei of the Langhans cells are much 

 larger than the nuclei of the syncytium. 



INTEGUMENT AND EPITHELIAL THICKENINGS. 

 The ectoderm is nowhere separated from the nervous system except over that part of 

 the brain tube between the anterior neuropore and the buccopharyngeal membrane. At 

 the anterior and posterior neuropores it is fused with the wall of the nervous system, and 

 everywhere else is connected by the neural crest to the mid-dorsal line of the neural tube. 

 The ectoderm is in general a 1-celled layer, but exhibits thickenings in certain definite 

 regions. In the dorsal portions of the body it forms a single layer of flattened cells and 

 presents a similar appearance ventrally over the heart region and at the sides of the body 

 close to the line of attachment of the amnion. Over the mesodermic somites its cells are 

 cubical, and this condition is maintained even in front of the somites, far up into the head. 

 Over the gill arches the ectoderm is much thickened and is 2-layered. These conditions 

 are exactly similar to those described and illustrated by Ingalls (1907) for an embryo 



