422 OUTLINES OF CHORDATE DEVELOPMENT 



contact, over practically its entire surface, with the uterine 

 mucosa. In other instances of central implantation and in 

 the eccentric and interstitial types, a part or even the whole of 

 the trophoblast becomes highly specialized, physiologically, 

 as the trophoderm (Minot), in which the cells proliferate rapidly 

 forming a layer of considerable thickness (Figs. 175, 176). 

 It is the function of the trophoderm to dissolve or digest the 

 uterine mucosa, with which it is in contact. The trophoderm, 

 probably through the action of specific enzymes, rapidly erodes 

 the uterine wall, and the blastodermic vesicle becomes either 

 partially or wholly embedded in the maternal tissue, so that 

 the embryo bears a relation to the maternal organism which 

 is quite that of an internal parasite. 



In most cases a part of the trophoblast is thus specialized 

 as trophoderm. In the rabbit, for example, the trophoderm 

 forms a horse-shoe shaped area just around the embryonic 

 rudiment, lateral and posterior to it; this region alone becomes 

 embedded in the uterine tissue, while the remainder of the 

 blastodermic vesicle, projecting into the lumen of the uterus, 

 remains covered with the relatively unmodified trophoblast 

 (chorion). In the spermophile (Rejsek) the trophoderm 

 forms a thickened mass, in the wall of the vesicle, opposite the 

 inner cell mass or embryonic rudiment. As the trophoderm 

 erodes the mucosa, the vesicle is carried down and partially 

 embedded in the uterine wall (Fig. 175). In forms like the 

 hedgehog, apes, and man, the entire trophoblast becomes 

 trophodermal (Figs. 153, 161). Here, then, the maternal 

 tissues on all sides of the vesicle are eroded, and the "ovum" 

 becomes completely embedded and surrounded by a mass of 

 dissolved tissue. 



The cells of the trophoderm very early begin to fuse together 

 forming either small masses, known as multinuclear giant cells, 

 or extensive protoplasmic masses known then as syncytia, or 

 better, the syncytiotrophoderm (syncytiotrophoblasi) (Figs. 175, 

 184). In the trophoblast, which is less intimately associated 

 with the maternal tissues, the cell boundaries usually remain, 

 and this is then distinguished as the cytotrophoblast. 



