CHAPTER II 

 CELL-DIVISION AND GROWTH 



1. CELL-DIVISION 



IN a future chapter we shall see that there is no necessary 

 connexion between segmentation and differentiation. Never- 

 theless, since cell-division is the first sign, or almost the first 

 sign, that a developing organism gives of its activity ; since, 

 moreover, cell-division accompanies the later processes of growth 

 and differentiation, we may briefly discuss what is known of 

 those factors which determine the direction of division in general, 

 and in particular the pattern of segmentation. 



We shall first presume that segmenting ova may be grouped 

 under several distinct types, as follows : 



1. T/ie radial type. Here the first division is meridional, the 

 second meridional and at right angles to the first, the third 

 equatorial or more often latitudinal and at right angles to 

 both the preceding, the fourth meridional and at forty-five 

 degrees to the first two, the fifth latitudinal. What is charac- 

 teristic above all of this type is, first, that four surfaces of 

 contact between cells meet in one line; for example, the four 

 surfaces between the first four blastomeres meet in the egg-axis, 

 while each pair of animal cells lies exactly over each pair of 

 vegetative cells after the third division; and secondly, the 

 blastomeres are radially arranged about the axis. This type has 

 been observed in Sponges (Sycon * (Schulze)), in Coelenterates, 

 in Crinoids, Holothurians, and Echinoids (Fig. 12) amongst the 

 Echinoderms, in Ectoproctous Polyzoa, in Amphioanw and the 

 Vertebrates, and in some Crustacea Cetocliilns (Grobben), 

 Lucifer (Brooks), Cyclops (Hacker), Branchipnt (Brauer) and 

 some Cirrhipedes. Certain of these cases present special 

 peculiarities. 



In Echinoids micromeres are formed at the vegetative pole 

 by the division of the fourth phase (Fig. 12, /). 



1 In Sycon the third cleavage is meridional, the fourth latitudinal. 



