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 placcntal 

 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 



O G 



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 Avidth. 



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 oyarium 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 (JBal&na mysti- 

 cetus\ harpooned in the month of August, a fo3tus 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. 



o 



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. 



