40 THE BIOLOGY OF TWINS 



in general have but little understood the essential 

 features of polyembryonic development in this species. 

 In the hope that this interesting piece of embryology 

 may be made quite definitely intelligible to a general 

 biological audience, I have had it re-illustrated by 

 Mr. Toda. The first six figures (stages I-VI) are 

 adapted from Patterson's photographs and figures; the 

 remaining stages represent material to which I have had 

 personal access. 



Stage /. The earliest embryos (Fig. 8). — The young- 

 est egg that has been found is in a rather late cleavage 

 stage, in which the embryonic cells, eleven in number, 

 form a small knot or inner-cell mass {icm) attached 

 to the inner Surface of the large, hollow sphere of non- 

 embryonic cells, the trophoblast {tr). The trophoblast, 

 as the name implies, has a purely nutritive function and 

 serves later to attach the vesicle to the walls of the 

 uterus. Of the eleven cells seen in the earliest egg, six 

 differ from the others in having larger nuclei. These 

 larger cells are destined to form the embryonic ectoderm. 

 The other cells form the endoderm. Such an egg is in 

 no way essentially different from any eutherian (higher 

 mammalian) egg, and is unquestionably at this time 

 without any visible indications of a prospective division 

 into four embryos. 



Stage II. The beginnings of gastrulation (Fig. 9) . — 

 The cells of the inner-cell mass have multiphed and 

 have spread out into a flat disk of one or two layers 

 in thickness. The distinction between ectoderm (ec) 

 and endoderm {en) is now evident in that the less numer- 

 ous endoderm cells are more deeply stained than the 

 ectoderm cells. The endoderm cells are also beginning 



