410 A HUMAN EMBRYO OP THE PRESOMITE PERIOD. 



sented by a solid cell mass 0.095 by 0.072 mm., undergoing cleft formation pre- 

 liminary to the development of the amniotic cavity. The embryo is surrounded by 

 a trophoblastic shell with an external diameter of 0.83 mm. and an internal diameter 

 of 0.44 mm. The trophoblast is partially differentiated into cytotrophoblast 

 (Langhans layer) and plasmoditrophoblast (syncytium) , the two partially merging 

 into one another. Peripherally, the plasmoditrophoblast forms irregular loops 

 inclosing large blood lacunae, about 12 in a single section, and gives the appearance 

 of eroding and engulfing the surrounding capillaries. There is no evidence of villi. 

 The interval between the trophoblastic ectoderm and the embryo is almost entirely 

 filled with a fine, granular deposit, through which pass simple fibroblastic strands. 

 These are arranged in a layer which lines the trophoblast, and are especially numer- 

 ous on one side of the embryo, between it and the trophoblastic ectoderm, appar- 

 ently the area of the body-stalk. 



The Bryce-Teacher ovum, both from its size and in its development, is to be 

 regarded as slightly older than the Miller specimen. It differs from the latter in the 

 following particulars: The trophoblast shell is surrounded by a necrotic decidual 

 area. The distinction between the cytotrophoblast and plasmoditrophoblast is 

 more definite, and the latter forms a more complicated network. There is no evi- 

 dence of any arrangement of the mesoderm into parietal and visceral lamellse it 

 still fills the blastocyst as a delicate tissue in the fine meshes of which the embryo 

 is suspended. The latter consists of two detached vesicles; the larger (diameter 

 0.168 mm.) is the amnio-embryonic vesicle with a wall of cubical cells; in the smaller 

 the yolk-sac (diameter 0.042 mm.) the cells are flattened. That these two vesicles 

 are completely detached and not in proper relation to each other would tend to 

 indicate that the specimen is not entirely normal. 



The beginning formation of villi can be seen in the ova described respectively 

 by Linzenmeier (1914) and by Peters (1899). Of these, the former is smaller and 

 probably younger; it is, however, distinctly older than the Bryce-Teacher specimen, 

 in respect to both the chorion and the embryo. The mesoblast is divided into 

 parietal and visceral layers, with a well-defined exoccelomic cavity. The parietal 

 mesoblast forms a continuous layer within the trophoblastic ectoderm, completing 

 the formation of the chorionic membrane. Short processes from its mesoblastic 

 layer project outward into the trophoblastic shell, producing the first villi; seven or 

 eight of these may be seen in a single section. The mesoblastic layer of the chor- 

 ionic membrane is continuous with the visceral layer of mesoblast surrounding the 

 embryo, at the seat of the future body-stalk. The embryo consists of two closely 

 opposed vesicles; the larger (greatest diameter 0.105 mm.) is clearly differentiated 

 into amniotic ectoderm and a thickened ectodermal embryonic plate; the smaller 

 constitutes the yolk-sac and is about half as large as the amniotic vesicle. It is 

 partially separated from the former by a layer of mesoderm. Whether an allantois 

 is as yet present could not be definitely determined from the author's description. 

 From the illustration the wall of the yolk-vesicle seems to consist of a very thin 

 membrane. 



