THE EARLY EMBRYOLOGY OF THE MOUSE 41 



probably also give rise to similar though somewhat smaller primary giant 

 cells. 



A second and much more numerous group of giant cells is quite probably 

 derived from the ectoplacental cone. Already at 5 days cells may be seen 

 growing down outside the trophectoderm from the region of the future cone 

 (Fig. 7). Later, when the embryo is surrounded by maternal blood, these 

 become long strands of cells extending down, within the blood or along the 

 inner surface of the decidua, from the cone towards the ventral extremity 

 of the egg cylinder." At first small, these cells increase in size and at 8 days 

 form a loose meshwork of large cells whose long protoplasmic processes 

 extend across the blood filled space between Reichert's membrane and the 

 decidua (Fig. 21). Other similar cells may be seen adjacent to the ecto- 

 placental cone. These are the secondary giant cells. At 8 days their 

 continuity with the cells of the ectoplacental cone is still quite obvious. 

 While this is the probable origin of the majority of the giant cells, the 

 possibility that at least some of them are derived from the decidua is not 

 ruled out. It should be pointed out that the division between primary and 

 secondary giant cells is partly arbitrary; the trophectoderm and the ecto- 

 placental cone are continuous structures, and cells from near the line of junc- 

 tion might be said to give rise to either type. One obvious function of the 

 giant cells is to anchor Reichert's membrane to the decidua. They quite 

 probably have other functions also, but what they are is uncertain. 



The third class of giant cells consists of the so-called symplasia. These 

 cells, individually conspicuous but never very numerous, are multinucleate 

 cells first appearing in the decidua adjacent to the embryo at 7 or 73^^ days. 

 The number of nuclei per cell is extraordinary, mounting into the dozens by 

 8 days. The nuclei are dark staining and closely packed. The origin and 

 function of the symplasia is uncertain. 



The seven somite embryo. — In embryos from genetically vigorous stock, 

 the seven somite stage is reached at about 8 days. As thereafter the embryo 

 begins a series of important changes, it will be useful to review here the 

 development attained at this point (Figs. 22 and 23). In sagittal section 

 the embryo is seen to form a letter S (facing to the left in Fig. 22) with the 

 head region convex, the trunk region concave towards the dorsal surface. 

 In transverse section, whereas the embryo was formerly conspicuously 

 cup-shaped with the ectoderm on the inside, it has now flattened out, in fact 

 in the regions of the fore- and hind-gut the entoderm has become the inner 

 layer. The neural groove, deep and well developed, is still open dorsally 

 though in the mid-trunk region the walls are quite close together. Cephalad, 



