490 ON INCREASE IN SIZE AND WEIGHT 



Table 65. Time-relations of early development (cont.). 



Molluscs 

 A. Richards' figures: 



[PT. Ill 



(Opisthobranch) 

 Planorbis 



Time Intervals 

 from in 



fertilisation minutes 



Haminea virescens 



, ' , 



Time Intervals 

 from in 



fertilisation minutes 



Fertilisation ... o 



1st cleavage ... 90 



2nd cleavage ... 165 



3rd cleavage ... 240 



4th cleavage ... 315 



5th cleavage ... 375 



E. G. Conklin's figures: 



90 

 75 

 75 

 75 

 60 



ASCIDIAN 



o 



85 

 136 



195 

 245 

 285 



Cynthia 



85 

 51 



59 

 50 

 40 



Fertilisation 



1st cleavage 



2nd cleavage 



3rd cleavage 



4th cleavage 



5 th cleavage 



6th cleavage 



7 th cleavage 



8th cleavage (218 cells) 



Hatched tadpole ... 



Time Intervals 

 from in 



fertilisation minutes 

 o 



40 



70 



90 



no 



130 

 150 



170 



190 

 310 



40 

 30 

 20 

 20 

 20 



20=" 

 20 

 20 

 120 



* Beginning of gastrulation. 



The inner significance of the length of embryonic life relative to 

 the life-span is most obscure. Some interesting remarks have been 

 made by Moulton, who has pointed out that over the whole life- 

 span, the chemical changes are much more intense in the earliest 

 periods, i.e. pre-natal and to a certain extent post-natal. This reminds 



