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 Hackel.) 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 accompan}ung 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.) 



