Reproduction in Multicellular Animals - 393 



Fig. 21-16. Usual position of the fetus shortly before 

 birth. 1, placenta; 2, umbilical cord; 3, uterus. (From 

 The Living Body, by Best and Taylor. Holt, Rinehart 

 and Winston, Inc.) 



grow out and surround the fetus almost com- 

 pletely (Fig. 21-15). 



In the placenta the maternal and embry- 

 onic bloods do not intermingle, but they 

 come into equilibrium with each other, be- 

 cause the two separate sets of capillaries lie 

 very close together over an extensive area. 

 Oxygen and other food substances are trans- 

 mitted from the maternal to the embryonic 

 blood, thus compensating for the fact that 

 mammalian eggs contain a very small supply 

 of yolk. Moreover, metabolic wastes, such as 

 urea and carbon dioxide, pass into the ma- 

 ternal blood and do not accumulate in the 

 blood of the embryo. 



At the end of the gestation period, deliv- 

 ery is initiated by the rupture of the embry- 

 onic membranes, in the region near the cervix. 

 This first liberates the amniotic fluid; and 

 shortly thereafter the fetus is forced through 

 the vaginal passage by a rhythmic series of 

 massive contractions of the uterine wall (Fig. 

 21-17). After the umbilical cord is tied and 

 cut, the infant must depend upon its own 

 nutritive organs. The afterbirth, which is de- 

 livered shortly after the fetus, represents a 

 part or all of the placenta. In man and some 

 other mammals, the whole placenta, includ- 



ing the maternal part, is expelled, but in 

 other mammals, the maternal tissues of the 

 placenta are retained within the womb. 



All mammals, except the monotremes and 

 marsupials, are called placentates, owing to 

 their common possession of the placenta (see 

 Chap. 32). The monotremes are oviparous 

 and display an essentially reptilian embryog- 

 eny. The marsupials, which include the 

 kangaroo and other pouched mammals, have 

 a fairly well-developed uterus, but no pla- 

 centa. Lacking adequate facilities for sustain- 

 ing the fetus until development is complete, 

 the marsupials deliver their young "prema- 

 turely." When born, the young marsupial is 

 deposited in a pouch that is an infolding of 

 the body wall. This pouch also surrounds the 

 mammary glands, so that the young are suck- 

 led and protected until a fuller development 

 is reached (Fig. 32-42). 



As is shown by the fossil record, mono- 

 tremes and marsupials were much more 

 prevalent in an earlier evolutionary era, but 

 a retrogression of the nonplacentates occurred 

 shortly after the placentates began to offer 

 serious competition. However, in Australia, 

 which was isolated from the mainland be- 

 fore the placentates became numerous, the 

 nonplacentates, especially the marsupials, 

 continue to prosper in considerable numbers 

 and varieties. 



ASEXUAL MULTIPLICATION IN 

 MULTICELLULAR ANIMALS 



Sexual reproduction is virtually universal 

 among multicellular animals, but many spe- 

 cies, especialy among the invertebrates, also 

 reproduce asexually — by methods that do 

 not involve fertilization. These asexual 

 processes include parthenogenesis — which 

 was discussed previously (p. 276) — as well as 

 fission and budding. 



Fission and Budding. The direct splitting 

 of an organism into two more or less equal 

 parts is called binary fission. However, some- 

 times a new individual arises from a rela- 

 tively small piece of the parent, and this 



