126 BIOLOGICAL LECTURES. 



deal more especially with the earliest stages of development 

 and with the factors which govern the histological differentia- 

 tion of the ovum ; and inasmuch as both these phenomena, in 

 those ova which have been most carefully studied, are asso- 

 ciated with a cleavage of the ovum into spherules standing in 

 definite relation to one another, inquiry has naturally been 

 aroused as to the laws or forces which determine the relation 

 which one cleavage plane shall bear to another. The cytogen- 

 etic method has revealed the fact that in the ova of any one 

 species, under normal conditions, the relations of the cleavage 

 planes to one another in the earlier stages vary only to a slight 

 degree, while the experimental method, on the other hand, has 

 shown that, under abnormal conditions, these relations may be 

 completely changed, and that in certain cases the cleavage 

 may even be temporarily suppressed, the nuclei only undergo- 

 ing division. External conditions, such as pressure, may then 

 interfere with the normal direction of the cleavage planes, but 

 what determines these normal directions ? 



Zoologists, however, have not been the first to consider this 

 problem, the earliest attempts at its solution having been made 

 by botanists, the greater deliniteness of the vegetable cell- 

 wall and the relative lateness of tissue-differentiation in plants 

 rendering the study of the problem apparently simpler than it 

 seems to be in the case of animal tissues; and in addition it is 

 noteworthy that the tendency to reduce the phenomena of life 

 to chemical and physical causes has been more marked in the 

 case of students of vegetable physiology, due perhaps to the 

 greater complexity of the vital phenomena exhibited by 



animals. 



A most suggestive discussion of the question has been given 

 by Sachs, the famous Wurzburg botanist, in a series of impor- 

 tant papers, the gist of his conclusions being, however, con- 

 tained in his lectures on vegetable physiology. He points out 

 in the first place a fact too often lost sight of by zoologists, 

 that there is a decided distinction between growth and cell- 

 division. In the animal kingdom the two phenomena are as a 

 rule somewhat intimately related, though a little consideration 

 will show that in it examples are to be found comparable to 



