924 



CARE AND NOURISHMENT OF THE DEVELOPING YOUNG 



and seventh days of uterine existence (i.e., 9 and 10 days after copulation) 

 (fig. 145I-L). The blastocyst eventually forms a much elongated attenuated 

 structure about 1 meter long. During the earlier portion of the free uterine 

 period the many blastocysts of the ordinary conceptual process in the sow 

 become spaced within the horn of each uterus, an intriguing process which 

 continues to remain baffling. From 10 to 13 days after copulation the blasto- 

 cysts experience the gastrulation processes (see figure 145M-R; and figures 

 208 and 209); from days 13 to 15 body form is developed gradually (fig. 

 242A-F) and the amnion and chorion are formed (fig. 242G). 



From days 14 to 17 the allantoic diverticulum grows rapidly (figs. 242G; 

 370C-D). At this time the chorionic vesicle as a whole shortens and becomes 

 much larger in transverse section. The yolk sac of the embryo of 16 to 17 

 days is greatly enlarged in relation to the size of the embryo, and the entoderm 

 at its distal end lies closely apposed against the chorionic ectoderm (figs. 

 242F; 370C). As the allantoic cavity expands, the yolk sac, relatively speak- 

 ing, contracts, and a relationship is established similar to that in figure 370D. 

 As the allantois expands its mesoderm comes in contact with the mesoderm 

 of the chorionic membrane and fuses with it (fig. 370E). This new layer 

 forms the chorio-allantoic membrane. The chorio-allantoic membrane becomes 

 folded into elongated folds which fit into similar folds of the uterine mucosa. 

 A relationship thereby is formed as shown in figure 373B. 



COTYLEDONS 



DOG, CAT, SEAL, GENET 



Fig. 378. External appearance of chorionic vesicles in various mammals. (A) Pig. 

 This placental type is called diffuse. (A') Enlarged drawing of small cotyledon or 

 areola. (B) Cow. Observe large cotyledons. This type of placenta is called cotyledonary. 

 (C) Brown bear. Special zonary placenta. (D) Dog, etc. Zonary placenta. (E) Rac- 

 coon, incomplete zonary placenta. (A, B-E, redrawn and modified from Hamilton, 

 Boyd and Mossman, 1947, Human Embryology, Williams and Wilkins, Baltimore.) 



