Embryology. 



135 



precisely similar series of events repeat themselves in 

 each of these two cells, thus giving rise to four cells 



FIG. 39. Segmentation of ovum. (After Hack el.) Successive stages 

 are marked by the letters A, B, C. D represents several stages in 

 advance of C. 



(see Fig. 29). It must now be added that such 

 duplication is continued time after time, as shown in 

 the accompanying illustrations (Figs. 39, 40). 



All this, it will be noticed, is 

 a case of cell- multiplication, 

 which differs from that which 

 takes place in the unicellular 

 organisms only in its being in- 

 variably preceded (as far as we 

 know) by karyokinesis, and in 

 the resulting cells being all con- 

 fined within a common envelope, 

 and so in not being free to 

 separate. Nevertheless, from 

 what has already been said, it 

 will also be noticed that this feature makes all the 

 difference between a Metazoon and a Protozoon ; so 

 that already the ovum presents the distinguishing 

 character of a Metazoon. 



FIG. 40. The contents of 

 an ovum in an advanced 

 stage of segmentation, 

 drawn in perspective. 

 (After Hackel.) 



