920 CARE AND NOURISHMENT OF THE DEVELOPING YOUNG 



Fallopian tube. As a result, when the developing human egg reaches the uterus 

 it is in the early blastula (blastocyst) condition (Chap. 6). The zona pellucida 

 or secondary egg membrane is still intact. The blastocyst remains free within 

 the uterus presumably for about four days. During this period, it becomes 

 separated from the zona pellucida (i.e., it hatches) and the blastocoelic cavity 

 of the blastocyst (blastula) subsequently enlarges greatly. The implantation 

 site for man (and also monkeys) under normal conditions is the mid-dorsal 

 or mid-ventral area of the uterus (Mossman, '37). The human embryo pre- 

 sumably begins to implant about 7 to 8 days after fertilization (Hertig and 

 Rock, '45). In doing so that pole of the blastocyst which contains the devel- 

 oping germ disc becomes attached to the uterine epithelium. As this occurs 

 the uterine epithelium becomes eroded in the area of immediate contact with 

 the blastocyst, and the epithelial cells of the trophoblast layer of the blastocyst 

 increase in number. As a result, the trophoblast tissue enlarges greatly in the 

 contact area (fig. 375A, F and G). During this process a change occurs in 

 the trophoblast cells for the external cells fuse together to form a syncytium, 

 the so-called syntrophoblast, while the inner trophoblast cells remain cellular 

 and form the cytotrophoblast (fig. 376A). The syntrophoblast presumably 

 acts as the invading tissue. {Note: the trophoblast tissue in figures 372A and 

 in 375 is shown in black.) As the syntrophoblast increases in quantity it comes 

 to enclose irregular spaces, the trophoblastic lacunae (fig. 375B-D). Simul- 

 taneously localized areas of the syntrophoblast extend outward to form the 

 primary villi (fig. 376A). These primary villi at first lack a mesenchymal core, 

 but soon they become invaded by the mesoderm of the somatopleure to form 

 the secondary villi (figs. 372B; 376B). At about 11 days, the developing 

 human embryo is completely inside of the uterine wall (fig. 3758). At 12 to 

 15 days (fig. 375C and D), the syntrophoblast has expanded considerably 

 and secondary villi begin to appear around the inner portions of the tropho- 

 blast (figs. 375D; 376B). Meanwhile (fig. 375D), some of the endometrial 

 tissue close to the invading chorionic vesicle, including blood vessels, is 



Fig 375 — Continued 

 blast tissue, the presence of trophoblastic lacunae containing endometrial residues, and 

 the formation of the secondary chorionic villi. (A-D, redrawn from Corner, 1944, Our- 

 selves Unborn, Yale University Press, New Haven, Conn.) (E) Placental relationships 

 at about 12 weeks. (Redrawn and modified from De Lee and Greenhill, 1943, The 

 Principles and Practice of Obstetrics, Saunders. Philadelphia.) (F) Early stage in 

 implantation of the monkey. Macaca mulatta, blastocyst about 9 days of age. (G) 

 Monkey blastocyst about 10 days. (H) Monkey blastocyst about 10 days. (I) Monkey 

 blastocyst 11 days. (J) Blastocyst of 13-day monkey embryo showing primary and 

 secondary implantation sites. (F-J redrawn from Wislocki and Streeter, 1938, Carnegie 

 Instit. Contributions to Embryology. Vol. 27, Contributions to Embryology. No. 160.) 

 (K) Placentae of Lasiopyga callitrichus. Observe that umbilical cord and its blood vessels 

 attach to the primary placental disc, while blood vessels are given off from the primary 

 disc to the secondary disc. (Redrawn from Wislocki. 1929. Carnegie Contributions to 

 Embryology, Vol. 20, Contributions to Embryology, No. 111.) 



