458 



ON INCREASE IN SIZE 



[PT. Ill 



had already averred qualitatively as early as 1828 that variability 

 was much more pronounced in the earlier stages than in the later ones. 

 Kupffer & Benecke; Keibel & Abraham; and Assheton afterwards 

 drew special attention to it. Fischel, however, measured the length 

 of the embryos, and found that the older they were the more regularly 

 they agreed together. This led him to conclude that regulating 

 influences came into play during development which brought about 

 a more synchronous course of growth and differentiation, and made 



6 7 8 9 10 n 

 •^ Number of Somites 



Fig. 61. 



the individual variations less and less obvious. Such a standpoint 

 would accord well with frequency-curves such as that of McDowell 

 and his collaborators for the mouse embryo (see Fig. 22), where 

 the range of weights on a given day is 4 or 5 times as large at the 

 beginning of development as it is at the end. His and Levi, 

 working on the development of the chick, came across the same 

 phenomenon. 



Fischel divided the total length of the embryo into a number of 

 constituent lengths, e.g. from the cephalic to the caudal end of the 

 somites and from the extreme cephalic end to the anterior blastopore 

 ("Darmpforte"). He distinguished 13 such lengths, and these he 

 measured in a large number of embryos, judging the age in each 

 case by the number of somites formed. As will be seen from Fig. 61, 

 the limits of variation considered as absolute maxima and minima 



