388 HISTORY OF THE HUMAN BODY 



foramen or may be absolutely closed. In the primates, in- 

 cluding Man, the connection is not as intimate as this, and the 

 ova occasionally escape into the ccelomic cavity, as is normally 

 the case among lower forms. Here, however, they usually 

 disintegrate and become lost, but in rare cases a fertilized 

 egg escapes in this way and may even attain considerable de- 

 velopment through the formation of a sort of placenta, at- 

 tached to the ccelomic wall. 



A uterus is more or less an adaptive organ, related to 

 Miiller's duct much as the crop is to the oesophagus; it is 

 primarily nothing but a localized enlargement and develops 

 whenever needed, in some cases appearing in a given form, 

 while absent in a closely related one. Thus in vivaparous 

 sharks (e.g., Squalus), the expanded lower portion of each 

 Miiller's duct becomes enlarged and forms a uterus in which 

 the embryos are retained until they reach practically the adult 

 form; and the same is true in the case of a certain salamander 

 (Salamandra atra). In none of these cases, however, is the 

 organ more than a container or brood cavity, and there is no 

 placenta formation or other direct connection between embryo 

 and uterine wall. The same is true of the lower mammals, 

 the marsupials and monotremes, in which there is no placenta, 

 and the young are produced in a very immature state ; but in 

 the higher, or placenta!, mammals, the wall of the uterus 

 becomes differentiated for the purpose of the nutrition of the 

 embryo, and thus becomes a definite physiological organ. 



There are numerous types of uterus among the mammals, 

 depending on the degree of fusion between the Miiller's ducts 

 of the two opposite sides ; and these types consequently pre- 

 sent a regular graded series between two distinct lateral uteri 

 and a single median one (Fig. no). 



In the first type of this series, that seen in monotremes, the 

 two ducts are entirely distinct from one another. They are 

 short, thick walled, and of rather large caliber, and may be 

 termed oviducts or uteri, according to the taste of the writer, 

 although the former term is more usually applied to them. 

 They open separately into a common urogenital sinus in close 



