vm] CONJUGATION 115 



swell, becomes vacuolated, and gradually assumes the form 

 of a nucleus. In eggs in which the egg nucleus is ready for 

 fertilisation before the spermatozoon enters, the sperm 

 nucleus (sometimes called the "male pronucleus") often 

 reaches the egg nucleus before it has reached its full size, 

 and in this case the two nuclei come into contact and unite 

 to form a single large zygote nucleus. If the sperm nucleus 

 is still small as compared with the egg nucleus this conjuga- 

 tion may have the appearance of the small sperm nucleus 

 being absorbed into the larger egg nucleus. In such eggs 

 the development of definite chromosomes only takes place 

 after the union of the two nuclei, and they then appear, of 

 course, in the double (somatic) number, half being derived 

 from each parent. The centrosome derived from the middle- 

 piece of the spermatozoon also only divides after the two 

 nuclei have come into contact, and the spindle of the first 

 segmentation division arises when the membrane of the 

 zygote nucleus disappears (PI. XII, J, K, L). 



In a large number of animals the polar divisions of the 

 egg only take place, or are only completed, after the 

 spermatozoon enters it. Sometimes the whole of the matura- 

 tion division is subsequent to the entrance of the spermato- 

 zoon ; often the first polar spindle is already developed, but 

 the entrance of the spermatozoon seems to provide the 

 necessary stimulus for the completion of that division and 

 for the formation of the second polar body. In other species, 

 again, the first polar body has already been formed but the 

 second division occurs only when the spermatozoon is 

 present. In all these cases the egg nucleus is not ready for 

 conjugation when the spermatozoon penetrates the egg 

 surface, and the subsequent behaviour of the spermatozoon 

 is therefore somewhat different from that described above. 

 In eggs of this kind the spermatozoon penetrates for some 



82 



