THE EARLY DEVELOPMENT OF THE MAMMAL 371 



egg resembles that of Amphioxus in size and deutoplasmic 

 relations, it exhibits almost none of the regularities of cleavage, 

 blastula formation, and gastrulation that we should expect to 

 be associated with a small, homolecithal ovum. 



The origin of the most fundamental modifications of the 

 sauropsid type of development lies in the replacement of the 

 intra-oval yolk-mass by a source of food and energy lying out- 

 side of the ovum and embryo, i.e., the maternal uterine walls, 

 and in the early and extensive relation between the embryo 

 and this new source of nutrition. 



In the following account of certain phases of mammalian 

 development, we shall not be limited to any single form through- 

 out, but shall describe in general, elementary terms, the mam- 

 malian type of development, using various forms as illustrations 

 of the topics considered. 



I. THE EGG AND ITS FORMATION 



1. The Reproductive Organs of the Female 



In. the Mammals there is always a single pair of ovaries, sus- 

 pended in the postero-dorsal region of the body cavity by 

 peritoneal mesovaria (Fig. 143). They are whitish, rounded 

 or ovoid bodies, of rather small dimensions (human, 3-4 cm. 

 long, by 2-3 cm. wide, by 0.7 1.2 cm. thick; rabbit, about 

 2 X 0.8 cm. The ovaries are not directly connected with the 

 gonoducts, although the openings of the oviducts are suspended 

 in the same peritoneal mesovaria, and are located very near to 

 the ovaries, so that the ova, when discharged from the ovary 

 pass only a short distance through the body cavity before 

 entering the oviduct (Fig. 143). 



The Miillerian ducts, or oviducts in the broad sense, are 

 muscular tubes, highly differentiated into three regions. The 

 upper or anterior portion forms the Fallopian tube or oviduct, 

 stricto sensu (Fig. 143). The inner end of this, where it opens 

 out of the body cavity near the ovary, is expanded and its 

 margin is drawn out into finger-like or fringe-like processes; 

 this is the infundibulum, ostium or fimbriated opening. The 



