476 



COLLEGE ZOOLOGY 



Calcaneus 

 Astragalus 



Cat 



digitigrade 



Horse 

 unguligrade 



Figure 348. Feet of mammals (left hindfoot). Man, generalized, with 5 digits (toes); the 

 entire sole rests on the ground (plantigrade). Cat, with 4 toes, heel raised; walks on digits 

 (digitigrade). Horse, most specialized, with only 1 digit (third); walks on hoof which covers the 

 end of his toe (unguligrade). Note that a sprinter runs on his toes to attain maximum speed 

 on the track. 



tiles. Their eggs are incubated by the female 

 and are hatched in the same manner as 

 those of reptiles and birds. The young of 

 marsupials, such as the Virginia opossum 

 (Fig. 351A), are born in a very immature 

 condition. At birth an opossum immediately 

 climbs by a hand-over-hand movement 

 through its mother's hair until it reaches the 

 pouch, where it remains attached to a nip- 

 ple for about 40 days. In the nine-banded 

 armadillo, a single egg gives rise to 4 young 

 (Fig. 351B), a phenomenon known as poly- 

 embryony; this rarely occurs in man. 



During their development the embr)'os of 

 mammals, as well as those of birds and 

 reptiles, produce two membranes: (1) the 

 amnion and (2) the allantois. Because of 

 the presence of these membranes, the mam- 

 mals, birds, and reptiles are often grouped 

 together as Amniota, while the amphibians, 

 fishes, elasmobranchs, and cyclostomes, 

 which do not possess these membranes, are 

 designated as Anamniota. 



The segmentation of mammals' eggs is 

 complete (except in monotremes) and takes 

 place either in the oviduct as in the rabbit, 

 or in the uterus as in the sheep. Figure 349 

 presents photographs of stages in the seg- 

 mentation of the rabbit's egg and develop- 

 ment of the human embryo; and Fig. 350 

 illustrates the early development of the 



human embr)'0 with embryonic mem- 

 branes. 



The placenta of some marsupials and all 

 the Eutheria arises in the following manner: 

 the young embr}'0 becomes connected to the 

 uterine wall by means of its outer epithelial 

 layer, now known as the trophoderm. This 

 later becomes coated wholly or in part on its 

 inner side by somatic mesoderm and consti- 

 tutes the membrane known as the serosa. 

 Later on, the splanchnic mesoderm of the 

 peripheral and distal part of the allantois 

 becomes applied to the serosa, and the two 

 structures constitute the embryonic mem- 

 brane called the chorion. The chorion de- 

 velops vascular villi which are lined with an 

 epithelium only one cell in thickness. These 

 thin-walled chorionic villi sink into pits 

 eroded in the wall of the uterus. Blood from 

 the maternal blood vessels oozes into these 

 pits. 



The connection of the chorion of the 

 fetus with the uterine wall gives rise to the 

 placenta, by means of which the nourish- 

 ment and respiration of the fetus are pro- 

 vided for in the body of the mother. Al- 

 though the blood of the fetus and that of 

 the mother are not in direct communication, 

 the membrane which separates them is so 

 thin that substances such as gases, food- 

 stuffs, and nitrogenous wastes pass through 



