THE EARLY HISTORY OF THE OVUM. 209 



the body of the mother, the latter being the prevailing method, 

 external impregnation occurring only in the cyclostomes and in 

 most teleosts and amphibia. Occasionally, as in some urodeles, 

 the spermatozoa are deposited in bunches (spermatophores) , 

 which are taken into the cloacal opening, effecting internal 

 impregnation ; or, as in most elasmobranchs, the ventral fins of 

 the males are modified into copulatory and intromittent organs 

 (claspers). In the birds the transmission of the sperm to the 

 female is effected by an apposition of cloacal openings, although 

 in a few birds a copulatory organ, the penis, is developed. In 

 the reptiles this structure acquires a greater development, and 

 reaches its extreme in the mammals. 



After the spermatozoon has penetrated into the egg, there 

 occurs, in all eggs accurately studied, certain phenomena which 

 constitute the process known as maturation. These chiefly 

 concern the nucleus, and are as follows : The nucleus ap- 

 proaches the surface of the egg and undergoes a normal divis- 

 ion, one of the resulting halves, together with a small amount 

 of protoplasm, being cast out of the egg as the first polar 

 globule. The nucleus now divides again ; but this division is a 

 reduction division, half of the chromosomes being cast out in 

 a second polar globule, while half sink back into the egg, which 

 now contains just as many chromosomes as does the sperma- 

 tozoon. These chromosomes now unite with those from the 

 head of the spermatozoon, forming the new nucleus of the egg 

 (the segmentation nucleus), which thus again contains the num- 

 ber of chromosomes characteristic of the species. Not until 

 this process is complete is the egg really impregnated and ready 

 for segmentation. 



The character of the segmentation varies according, among 

 other things, to the amount and distribution of the food yolk 

 in the egg. This substance is inert, and its presence interferes 

 with the living and active protoplasm. Were no deutoplasm 

 present, the egg would divide in a regular and equal manner, 

 and the resulting cells would be equal in size ; and the same 

 would be true, other things being equal, were the deutoplasm 

 small in amount and evenly distributed through the protoplasm. 

 Such eggs do occur in the non-vertebrate groups, but none are 



