732 ANATOMY OF VERTEBRATES. 



cast off at its birth, are diversified in both form and structure 

 within the limits of a subordinate natural group of placental 

 Mammals. According to Sharpey, 1 the outer surface of the 

 chorion is reticularly ridged, like the inner surface of the human 

 gall-bladder, but in a finer degree. The inner surface of the 

 uterus exhibits fine low ridges or villi, not reticulating quite so 

 much (qu. with more open meshes?). The chorion, also, presents 

 a band, free from villi, running longitudinally along its concavity, 

 and there is a corresponding bald space on the surface of the 

 uterus. The ridges of the chorion start from the margins of the 

 bald stripe, and run round the ovum. The vitellicle is fusiform. 



The species of the order Bruta are uniparous as a rule : the 

 foetus attains a relatively large size, and the pelvis has a corre- 

 sponding width. 



§ 402. Development of Mutilata. — The Cetacea are uniparous, 

 and still more remarkable for the large proportional size of the 

 young, at birth : its membranes extend from the division of the 

 uterus corresponding to the impregnated ovarium into that of the 

 opposite side. A general short verrucose villosity of the chorion 

 intus-suscepted by corresponding alveolar modifications through 

 decidual outgrowths of the lining substance of the uterus performs 

 the placental function : the structure is least developed at the 

 terminal blind ends of the chorionic sac, which are almost smooth, 

 and, in the degree in which the diffused placenta is thus inter- 

 rupted at the poles, it may be said to be broadly zonular. The 

 amniotic sheath of the umbilical cord is beset with small pedun- 

 culate corpuscles. 2 In flensing a female Whale (Balana mysti- 

 cetns), harpooned in the month of August, a foetus escaped from 

 the vulva: it measured 5 feet 4 inches in length ; and was pro- 

 bably far from the full time. No bony pelvic cincture offers a 

 mechanical obstacle to the birth: and the exigencies of a hot- 

 blooded air-breathing animal sent from the warm womb into — it 

 may be — an arctic sea, call for muscular powers equal to the evo- 

 lutions needed for maintaining contact with the nipple, and coming 

 to the surface to breathe. 



Of the foetal membranes of the Sirenia nothing is known. 



§ 403. Development of Unyulata. — Here no envelope of the 

 ovum is superadded to the hyalinion (' zona pellucida '). "With 

 this for the outer covering the ovum enters the uterus : it is im- 

 pregnated in the oviduct, where it meets the spermatozoa; the first 

 stages of cleavage go on there, and the germ-mass is completed in 

 the uterus. In this process the hyalinion thins away, and finally 

 1 As quoted in cclxx". p. 112. 2 xx. vol. v. p. 200. 



