74? SURVEY OF THE REPRODUCTIVE PROCESS 



original ovum, owing to the obliteration of tlie primordial 

 wall of the latter, and the subsequent formation of a new 

 limiting membrane — the vitelline — nearer the w^all of the 

 ovisac* The supplementary material does not undergo 

 segmentation, and goes under the name of food-yolk ; it is 

 evidently provided to secure a larger supply of nutriment 

 within the e<>;oi:, in cases where none is to be obtained from 

 without during the development of the embryo. It is not 

 present in the Mammalia, where the fsetus is nourished 

 through the placenta, nor in cases among the lower orders 

 in which the embryo is speedily set free as a larva. 



It is to diversities in such secondary points as those last 

 alluded to, that the infinite variety is due, which the 

 Animal Kingdom presents in the details of the process of 

 reproduction. In the points more essentially connected 

 with the formation and fertilization of the germ, there is, 

 as has been mentioned, an essential sameness throughout, 

 for the phenomena which at first sight appear most eccentric 

 — those connected with the alternation of generations — do 

 not arise from anomalies in the sexual process itself, but 

 from the interpolation of an independent process of Mono- 

 genesis, at different periods in the life history of the species. 



It is the relation between these two associated processes 

 Avhich will form the principal subject of attention in this 

 summary of the modifications of reproduction in the more 

 important groups of animals ; the other peculiarities of the 

 function will be alluded to only in so far as they help to 

 illustrate this point. 



§ 2. REPRODUCTION IN THE PROTOZOA. 



Following the same ascending order as in the last 

 chapter, we are met at once on the threshold of the Animal 

 Kingdom with a form of life to which we are not yet in a 



^ Thomson in Cyc. Anat. and Pliys., Part LIV., p. 77. 



