EMBRYOLOGICAL INVESTIGATIONS 255 



normal proportion of sodium chloride, the eggs did 

 not segment, yet that division of the nucleus 

 occurred : unsegmented eggs were in this way 

 obtained with from four to as many as thirty 

 nuclei. 



It is at least possible that some of the variations 

 in the normal process of segmentation mentioned 

 above as observed in other animals may be due to 

 changes in the composition, or perhaps of the 

 temperature of the water in which the eggs were 

 developing. Herbst's interesting experiments on 

 the modifications in the development of Echinoderm 

 larvae produced by, the addition of potassium or 

 lithium salts to the sea-water in which they were 

 contained, led him to conclude that the influence on 

 the developing ova was not directly chemical, but 

 was due to the altered osmotic pressure of the sea- 

 water. The field of enquiry thus opened up is a 

 most promising one, and is certain to receive the 

 attention of embryologists in the immediate future. 

 We are at present ignorant of the causes which 

 determine the division of a cell, or the segmentation 

 of an egg ; but we shall have made an appreciable 

 step towards a right understanding of the pheno- 

 mena when we have determined with precision in 

 what way they can be modified by slight, but known, 

 alterations in the environment. 



Variations occur in the later as well as in the 

 earlier stages of development ; and the differences 

 between allied genera or species, or even between 

 closely related individuals, may be very marked, 

 Among Ccelenterates, for example, the mode of 



