PT. m, SECT. 2] ON INCREASE IN SIZE AND WEIGHT 369 



It is obvious that the growth of an embryonic organism can be 

 measured in many ways besides that of increasing weight. Its en- 

 larging dimensions in various directions of space can be measured, 

 or its volume, or the quantity of various constituent substances. More 

 will later have to be said about the way in which these different 

 quantities may be thought of as fitting in together and changing 

 with age. But the simplest manner of representing growth will 

 probably always remain the measurement of the increase in weight 

 of the total mass, and it is this which is now to be considered. The 

 relation of this factor to the age and the length of the foetus is a 

 point of capital importance to the chemical embryologist in the 

 knowledge of his material. It is true that the data are fragmentary 

 enough, restricted as they are almost entirely to various mammals 

 and the chick. 



2'2. The Existing Data 



Cephalopods. 



Octopus. A remarkably complete set of data for the embryonic 

 growth oi Sepia is given by Ranzi, and this is almost all we have as 

 regards invertebrate development. 



Insects. 



Silkworm. Luciani & LoMonaco have studied the curve of growth 

 through the successive moults in the larval condition, but, in spite 

 of their work and of many other researches on the silkworm larva 

 and ^gg, I cannot find any in which the increasing weight of the 

 embryo itself has been measured prior to hatching. 



Fishes. 



Trout. Weighings of fish embryos have been exceedingly few in 

 number, owing to the smallness of their size and the difficulty of 

 separating them from the yolk. Kronfeld & Scheminzki, however, 

 have made some estimations of the increase in weight of trout 

 embryos, and their figures, together with those of Gray, are shown 

 in Table i of Appendix i. 



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