470 ON INCREASE IN SIZE [pt. iii 



cells from embryos killed at a definite time. Perhaps it would be 

 found that the younger the embryo, the longer would its component 

 cells remain viable after its demise as a complete whole. Thus 

 Bianchini & Evangelisti found that in guinea-pig and rabbit foetuses 

 the cells of tissues were cultivatable as long as 78 hours after the 

 death of the whole body. And Bucciante, who incubated hen's eggs 

 for 6-12 days and then placed them at o or 15°, found that in 

 vitro cultivation was still possible for as long as 25 days afterwards 

 in the case of epithelial cells, though leucocytes and liver-cells only 

 retained viability for 3 or 4 days. Cells of other tissues occupied 

 various intermediate positions. 



2-13. Incubation Time and Gestation Time 



So far, the total time taken in embryonic growth has not been 

 considered at all. It was indeed said, when the work of Donaldson, 

 Dunn & Watson was being considered, that the act of birth or 

 hatching seems sometimes to have little influence on the course of 

 growth, but as Brody and Schmalhausen have shown, it more com- 

 monly aflfects profoundly the rate of increase of size. And it does set 

 a term to the embryonic period (I make no distinction between em- 

 bryonic and foetal period, as is the custom with some authors), forming 

 at least a convenient index to show whether the growth of the embryo 

 is being accelerated or retarded by external influences. It is therefore 

 important to know the normal incubation or gestation time for as 

 many animals as possible. These are collected together in Tables 60, 

 63 and 64. It can easily be seen that there is some difference between 

 the estimates of gestation and incubation times in the same animal 

 by different workers, but these discrepancies can only be cleared up 

 by more extended observations. 



When we come to consider, however, the nature of the law which 

 must presumably govern the length of embryonic life we meet with 

 a remarkable degree of obscurity. Roughly speaking, the common- 

 sense rule that the larger an animal is, the longer its embryonic life 

 must be, is borne out by the figures. Thus in Table 63, where the 

 weight of the adult mammal is seen to vary between 0-014 kilo 

 and nearly 4000-0 kilos and the gestation time between 21 and 600 

 days, the larger mammals have the longest gestation times, although 

 there are several cases where animals of the same weight have 

 different gestation times (e.g. the pig and the deer) and animals of 



