430 THE PHYSIOLOGY OF REPRODUCTION 



coitus, and the blastodermic vesicles remain free till the seventeenth 

 day. Then the attachment to the mucosal surface begins, and it is 

 completed by the thirtieth day (Assheton). After the ninth day, 

 when the prochorion ruptures, the trophoblast comes in contact with 

 the uterine epithelium. Apparently, as the result of this, the 

 absorption of nutriment is easier, and the blastodermic vesicle 

 increases rapidly in size so as to fill the uterine horn, or both horns 

 if only one embryo is present. 



Certain changes occur in the mucosa before attachment. The 

 leucocytes, which in the non-pregnant uterus are situated at the 

 base of the lining epithelium, increase in number and penetrate 

 between the epithelial cells. The glandular sacs, situated at the 

 junction of the branches with the main ducts, expand greatly and 

 actively secrete. It is generally held that the surface epithelium 



FIG. 115. Section through the uterine and embryonic parts of a cotyledon of the 

 sheep at the twentieth day of pregnancy. Folds in the trophoblast fitting 

 into sulci of the cotyledonary burr. (Assheton.) 



mes, Mesoblast ; tr, trophoblast ; tu, degenerated uterine epithelium ; 

 str, uterine stronia. 



is not destroyed, but Assheton has shown that on the cotyledonary 

 burrs it is distinctly degenerated by the seventeenth day, and he 

 lias also brought forward strong evidence that it is not subsequently 

 regenerated, but is replaced by bi nucleate cells of the fetal ectoderm. 

 In the cotyledonary areas of the trophoblast, villi are developed 

 as buds of epiblast, which afterwards contain cores of mesoblast with 

 branches of the allantoic vessels (Fig. 115). They fit into depressions v 

 or crypts on the surface of the cotyledons, increase in length, and 

 branch in different directions. Whether they literally grow into 

 the maternal tissues either mechanically or by a phagocytic action 

 is uncertain. 1 It seems more likely that very little, if any, further 

 penetration occurs, but that the sub-epithelial tissue swells and k< <]> 

 pace with the villi as they increase in length. The crypts, if their 

 lining cells really belong to the fu?tal ectoderm, are not secretory, 



1 At this stage Aasheton did not observe any actual engulfment of cells, but 

 considered that nutriment might be transmitted by fine processes of the binu- 

 cleate cells which united with similar processes of the connective tissue cells of 

 the mucosa. 



