460 



ON INCREASE IN SIZE 



[PT. Ill 



10-0 



the earlier the stage the more chance there will be for individual 

 differences in growth-rate to reveal themselves, while in later develop- 

 ment these will be equalised, adjusted, and compensated by a process 

 of self-regulation. Schmalhausen does not say, however, what this 

 process of self-regulation is, and its nature, indeed, offers one of the 

 most interesting problems in embryology. We shall refer to it again 

 in the chapter on hormones. Schmalhausen suggests that Philip- 

 tschenko's results might be explained by the extremely small time 

 elapsing between the formation of the various anlages in animals 

 with ultra-rapid incubation times such as some insects. Owing to the 

 high instantaneous growth-rates in the earliest stages, a minute 

 difference in time of formation of an anlage or a minute difference 

 in its initial size will make a very large difference between one 

 individual and another. 



2*12. Explantation and the Growth-promoting Factor 



We must now return to consider further the essential nature of 

 the embryonic growth process. Some reference has already been 

 made to Schmalhausen's findings 

 with regard to spurts of growth 

 and the extent to which all organs 

 are affected simultaneously by 

 the growth-impulse. But what is g ?• 

 this growth-impulse? What is its .£ e-o 

 essential nature? It is no longer ^ g.o 

 possible to say that we have no J ^ 

 idea, for there are a number of 

 very significant observations re- 

 garding it arising from the study 

 of tissue culture in vitro outside 

 the living body. Ephrussi has 

 brought forward evidence sug- 

 gesting that growth-rate may be a genetic factor. The study of the 

 effect of temperature on the growth-process has also been fruitful to 

 a certain extent in answering this question, and its results will shortly 

 be described. 



In the first place, it is natural to enquire whether the growth-rate 

 found to hold for the entire embryo growing under its normal con- 

 ditions would hold for parts of it isolated and growing in tissue 



4 & 



6 7 8 9 10 1112 13 14 15 16 17 18 

 Incababion age 



Fig. 62. 



