544 ON INCREASE IN COMPLEXITY [pt. iii 



embryogenic processes is seen in the interesting work of Twitty, who 

 studied the development and the nature of the action of the ciha on 

 the skin of amphibian embryos, previously described by Assheton and 

 Woerdemann. He found that the polarity of the ciliary cells is deter- 

 mined during the closure of the neural folds, for cilia grafts rotated 

 1 80° before that stage beat in the same direction as the cilia of the 

 adjacent ectoderm, but cilia grafts transplanted later retained their 

 original direction of waving. Thus the determination here (in 

 Amblystoma punctatum) occurred much later than the main point of 

 chemodifferentiation (see p. 575). Now in embryos allowed to 

 develop at low temperatures, Twitty found that this ciliary polarity 

 appeared at a much earlier stage. Evidently the determinative 

 process had been thrown out of gear with the morphological ones 

 by the cold, and the two were proceeding more or less independently. 

 Again, certain treatments, as is well known, inhibit segmentation, 

 and cause the production of cilia, while histogenesis and organo- 

 genesis are easily separable as is seen in teratological experiments 

 (Ranzi) and in explantation work (Hoadley)*. 



3*2. Differentiation-rate 



Under certain conditions then, such fundamental processes as 

 growth and differentiation can be shown to be proceeding indepen- 

 dently of each other. That their normal velocities differ might or 

 might not be the case. All who have had any practical contact with 

 embryology know that there is much more difference in shape and 

 form between a chick embryo of the 2nd day and one of the 5th than 

 there is between one of the 12th day and one of the 15th. But the 

 difficulty has always been to evolve some method of measuring change 

 in shape — a more elusive entity than change in weight. Murray, 

 however, has made an admirable attempt in this direction, choosing as 

 the organ for investigation the heart, which, as we have already seen 

 in the work of Schmalhausen, appears to keep pace exactly as regards 

 growth with the embryo as a whole. He tested this by measurements of 

 the surface area of the organ, calculating the percentage growth-rate 

 of the heart from Cohn's work with a projectoscope and a planimeter. 

 The result was one of the usual descending curves which closely 

 resembled that for the embryo as a whole, and the log. surface area 

 of the heart/log. age of embryo relation was also a straight line. 



* Thus Waddington, working with chick embryos cultivated in vitro, found that the 

 i2-somite stage was much smaller than the corresponding stage in the egg. 



