296 



THE EVOLUTION THEORY 



megcdocephala), which has become famous through the classical 

 observations of Ed. van Beneden. Many favourable circumstances 

 unite in this case to make the essentials of the process "clearly recog- 

 nizable. Fertilization takes place within the body of the female, in an 



b.M ' 



Fig. 75. Process of fertilization in Ascaris megalocephala, the thread-worm of the 

 horse, adapted from Boveri and Van Beneden. A, ovum in process of the first directive 

 division ; Bki, first polar body; sp, spermatozoon vyith tvsro chromosomes in its nucleus, 

 attaching itself to the ovum, and about to penetrate into it ; a protrusion of the egg- 

 protoplasm is meeting it. B, the second directive division has been completed; Bks, 

 the second polar body ; Eik, the reduced nucleus of the ovum. The first polar body 

 (life I ) has divided into two daughter-cells, spk; the nucleus of the spermatozoon 

 remains visible with its two centrospheres (csph). C, the sperm nucleus (.,5 k) and the 

 ovum nucleus (? k) have grown, each has two loop-like chromosomes ; only the male 

 nucleus has a centrosphere, which has already divided into two (csph). D, the two 

 nuclei lie apposed between the poles of the nuclear spindle. S, the four chromosomes 

 have split longitudinally ; the spindle for the first division of the ovum (the segmenta- 

 tion spindle, fsp) has been formed. F, divergence of the daughter-chromosomes towards 

 the two poles ; division of the ovum into the first two cleavage cells or embryonic 

 cells. 



