I.] IMPREGNATION. 17 



hours, during which time the shell acquires its normal 

 consistency. At the time of laying it is expelled from 

 the uterus by violent muscular contractions, and passes 

 with its narrow end downwards along the remainder of 

 the oviduct, to reach the exterior. 



Impregnation. This process occurs in the upper 

 portion of the oviduct; the spermatozoa being found 

 actively moving in a fluid which is there contained. 



We have as yet, as far as the fowl is concerned, no 

 direct observations concerning the changes preceding 

 and following upon impregnation ; nor indeed concern- 

 ing the actual nature of the act of impregnation. 



In other types however these processes have been 

 followed with considerable care, and the result has been 

 to shew that prior to impregnation a division of the 

 ovum takes place into two very unequal parts. The 

 smaller of these parts is knoMTi as the polar body, and 

 plays no further part in the development. In the 

 course of the division of the ovum into these two parts 

 the germinal vesicle also divides, and one part of it 

 enters the polar body, while a portion remains in the 

 larger segment which continues to be called the ovum, 

 and is there known as the female pronucleus. Im- 

 pregnation has been found to consist essentially in 

 the entrance of a single spermatozoon into the ovum, 

 followed by the fusion of the two. The spermatozoon 

 itself is to be regarded as a cell, the head of which 

 corresponds to the nucleus. When the spermatozoon 

 enters the ovum the substance forming its tail becomes 

 mingled with the protoplasm of the latter, but the head 

 enlarges and constitutes a distinct body called the male 

 pronucleus, which travels towards and finally fuses with 

 P. & B. 2 



