342 THE EABBIT. 



nucleated cell, of epithelial origin. This ovum, after being 

 fertilised by a spermatozoon from the male, undergoes cell 

 division or segmentation, which, as in the bird, is effected during 

 the passage of the ovum along the oviduct. On reaching the 

 uterus, which is simply the distal, enlarged end of the oviduct, 

 the ovum halts, becomes fixed to the wall of the uterus, and there 

 remains during all the further stages of development, up to the 

 time of birth. 



The actual details of development of the rabbit embryo, the 

 mode of formation of its various organs, and even the propor- 

 tions of the several parts and the order of their appearance, are 

 essentially the same as in the chick. The most noteworthy 

 point of difference is the exceeding slowness with which the 

 earlier stages are passed through in the rabbit. 



The entire period of development of the rabbit, from the 

 discharge of the ovum from the ovary, to the birth of the 

 young animal, occupies thirty days. Of this time the first three 

 days are spent by the egg in passing down the oviduct, and in 

 undergoing segmentation ; and it is not until the seventh day 

 that the first trace of the embryo appears ; the rabbit's ovum 

 at the end of the seventh day being in a condition closely 

 corresponding to that of the chick about the end of the first day 

 of incubation (c/. Figs. 107 and 143). From tliis time, develop- 

 ment proceeds much more rapidly, at about the same rate as 

 in the chick, a twelve-day rabbit embryo (Fig. 161) correspond- 

 ing closely in structure, and in actual size, with a five-day chick 

 embryo (Fig. 115). 



The eggs of the three animals described in the previous 

 chapters differ very greatly in size. That of Amphioxus has a 

 diameter of only 0-1 mm. ; the frog's egg measures about 

 1'75 mm. ; and the yolk, or true ovum, of the hen's egg about 

 30 mm. ; i.e. in round numbers the frog's egg is 5,000 times, 

 and the hen's egg 27,000,000 times the bulk of that of Amphi- 

 oxus. It has been already shown that the dimensions of the 

 egg are chiefly governed by the amount of food-yolk which it 

 contains, and that the size of the young animal on leaving the 

 egg is directly dependent on the amount of this food material ; 

 the tadpole on hatching being far larger than the Amphioxus 

 larva, while the chick on leaving the egg vastly exceeds the 

 tadpole in bulk. 



