POLYEMBRYONIC DEVELOPMENT IN TATUSIA 617 



which is found an account of the late stages of development 

 (Newman and Patterson '10). 



The differentiation of the embryo within the tube is soon 

 made evident by the elevation of the neural folds to form the 

 neural tube, and by the cutting off of paired mesodermic so- 

 mites. As these changes in the embryo are progressing, the tube- 

 like amnion is rapidly undergoing modification. First, it devel- 

 ops a finger-like prolongation which extends back beyond the 

 posterior limit of the embryo. Then there follows a rapid ex- 

 pansion of that portion of the amniotic tube which is occupied 

 by the embryo, the anterior portion of the tube remaining small, 

 and, together with the common amniotic vesicle, is destined to 

 degenerate and disappear. 



During the expansion the amnion at first assumes a cigar-like 

 shape, and the amnia of the four embryos soon come to fill the 

 entire cavity of the chorionic vesicle, each embryo and its mem- 

 branes occupying one quadrant of the vesicle. In the mean 

 time, each embryo has become constricted from the extra-em- 

 bryonic parts by the development of the characteristic head, 

 tail, and lateral folds, and retains its connections with the 

 chorion by means of a typical umbilicus. It thus floats quite 

 freely within the amniotic fluid. 



During all of these changes the paired condition of the embryos 

 is perfectly distinct, and, in the final stages of development, 

 expresses itself in the arrangement of the embryos into right- 

 hand and left-hand pairs, as is evidenced by the umbilical con- 

 nections with the placenta (fig. 79). 



IMPLANTATION OF THE OVUM AND PLACENTATION 



The more general features of the late phases of placentation 

 have been presented in another paper, but most of the material 

 for the study of the early development of the placenta has but 

 recently been secured. The nature of this paper does not call 

 for a detailed account of this process, and even though it did, 

 it would not be possible to write it in full at this time, since one 

 or two of the critical stages have not yet been been obtained. 



