THE EGG 



embryo soon becomes a hollow sphere, with a little mass of 

 cells at one side (Plate XI, J and K), This inner cell mass 

 is to become the embryo proper ; the remainder of the spher- 

 ical cyst becomes the embryonic membranes. 



During the first few days after arrival in the uterus the 

 embryos are free and unattached. In animals bearing only one 

 infant at a time, as usual in the human species, the embryo 

 simply settles down somewhere on the inner wall of the uterine 

 cavity. Species bearing several young have long uterine horns 

 to provide room for them all. In such animals the free embryos 

 must be moved along the uterus, by a kind of squirming move- 

 ment of the uterine walls, until they are spaced at regular 

 intervals. 



Implantation. Attachment or implantation begins, in most 

 species, about the 7th day, but in some (e.g. the sow) as late 

 as the 13th. The nature and extent of the attachment of 

 infant to mother, though fundamentally the same in all mam- 

 mals, varies a good deal, in its structural character, in the 

 various orders of the Mammalia. In the ungulates (hoofed 

 animals), for instance, the attachment is not very intimate. 

 In the sow the voluminous membranes of the embryos are 

 simply apposed to the inside of the uterus, like a string of 

 crumpled bags fitted inside the long uterine horns, and the 

 embryos get their nutritive fluids and the oxygen they breathe, 



Plate XII. Implantation of the embryo in the primate uterus. A^ embryo of 

 Rhesus monkey (Carnegie C. 610) in the blastocyst stage, 9 days old, just 

 settling down on the lining of the uterus. The little white spot in the center is 

 the "inner cell mass," destined to become the embryo proper. Magnified 60 

 times. B, human embryo about 12 days old (Carnegie 7700) which has burrowed 

 into the uterine lining. Magnified 12 times. C, section of a very similar human 

 embryo (Carnegie 7802) showing how it lies within the endometrium. The 

 glands of the uterus are in a state of progestational proliferation under the 

 influence of the corpus luteum hormone (see Chapter V), Magnified 10 times. 

 D, portion of same section magnified 30 times, to show the embryo proper 

 {emh.) and the placental villi, which are beginning to grow out from the en- 

 velope (chorion) of the embryo. A by courtesy of G. L. Streeter, C. H. Heuser, 

 and C. G. Hartman ; B, C, and D, by courtesy of A. T. Hertig and John Rock. 

 Photographs by Chester Reather. 



