430 



THE PHYSIOLOGY OF REPRODUCTION 



coitus, and tho blastodeniiic vesicles remain free till the se\'enteenth 

 day. Tlien the attachment to the mucosal surface l)egiiis, and it is. 

 completed liy the tliirtieth day (Asslieton). After tlie ninth day, 

 when the prochorion ruptures, the tropliohlast comes in contact with 

 the \iteriiie epithelium. Apparently, as the result of tliis, the 

 absorption of nutriment is easier, and the Idastodernnc vesicle 

 increases rapiiUy in size so as to fill tlie iiterine horn, or lioth horns 

 if only one embryo is present. 



Certain clianges <.iccur in tlie mucosa Ijefore attachment. The 

 leucocytes, wliicli in tlie non-pregnant uterus are situated at the 

 base of the lining epithelium, increase in number and penetrate 

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

 jiinction df tlie branches witli the main ducts, expand greatly and 

 activeh' seciete. It is geneially held that the suii'ace epithelium 



FitJ. 11.5. — Section tLrough the uterine and embryonic parts of a cotyledon of the 

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

 into sulci of the cotyledonary burr. (Assheton.) 



nies, Mesoblast ; tr, trophoblast ; ux, degeneiated uterine epithelium ; 

 str, uterine stioma. 



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

 binrrs it is distinctly degenerated l)y the seventeenth day, and he 

 has also lirouglit forward strong evidence that it is not subsecpiently 

 regenerated, but is replaced bj- 1)inucleate cells tif the ftetal ectoderm. 

 In the cotyledonary areas of the troplioblast, villi are developed 

 as buds of epi1;ilast, whicli afteiwards contain cores of mesoblast with 

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

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

 Ijranch in different directions. Whether they literally grow into 

 the inatenial tissues either mechanicall}- or liy a phagocytic action 

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

 penetiation occurs, l)Xit that the suli-epithelial tissue swells and keeps 

 pace with tlie villi as they inciease in length. The crypts, if their 

 lining cells really belong to the fo-tal ectoderm, are not secretory, 



' At this stage Assheton did not observe any actual engulfment of cells, but 

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

 cleate cells which united with siinilai- piocesses of the connective tissue cells of 

 the mucosa. 



