234 Professor Clark on a Case 



^tli of a line. The human ovum is still less than this: and, in 

 general, the size of an ovum seems to be inversely proportional 

 to the future evolution of the order. It may be mentioned, as 

 bearing upon the subject of this paper, that two ova have been 

 occasionally observed in one graffian vesicle, in the dog. The size 

 of the ovum depends upon the yolk, whose magnitude varies ac- 

 cording as it is intended to supply nutriment for the whole of 

 foetal life, or for a portion of it only, and this seems to affect the 

 extent of the germinative membrane. For in birds the blastoderma 

 occupies at first a small portion only of the surface of the yolk. 

 In mammalia, on the contrary, it appears from the beginning to 

 invest the whole. This at least is Burdach's opinion, though not 

 Von Baer's. 



Constituted as above described, the ovum of mammalia escaping 

 from the ovary, is received into the Fallopian tube. In that organ 

 it does not receive any new parts. It absorbs however, as it passes, 

 the fluid of the tubes, and so increases in size. The great changes 

 which it undergoes are afterwards effected in the uretus. 



In the different classes of animals the successive changes in the 

 ova are not effected in corresponding parts of their productive ap- 

 paratus. In the lower forms of life the changes of the egg are 

 begun and completed in the ovary. In invertebral animals the 

 egg is so far completed in the ovary, that it receives only its ex- 

 ternal shell in the oviduct. In oviparous vertebral animals, as in 

 mammalia, the yolk and the two membranes are alone formed in 

 the ovary. Their egg however receives in the oviduct the suc- 

 cessive deposits of albumen with its membranes, the membrane 

 of the shell, and the shell itself: whilst in mammalia the parts 

 analogous to the membrane of the shell, and the shell, are com- 

 pleted in the uterus. Tims according as the place of the animal 



