1896.] 



Embryology. 



769 



extend quite to the level of the nuclei, the position of the latter in the 

 figure having been taken from subsequent sections in the series. This 

 mass, however, is not to be regarded as "debris," but its presence is 

 due to a definite cause to be explained later. The remainder of the 

 section is occupied by homogeneous protoplasm filled with rather small 

 yolk granules, and surrounded by the thin, transparent vitelline mem- 

 brane. 



Under a high power (Fig. 40) the wrinkles appear as deep sinuses 

 extending obliquely into the protoplasm, and bordered by a thick layer 

 of pigment. These sinuses are angular and irregular in outline, and 

 often cantract at their inner ends into long, narrow slits, with rather 

 distinct walls. We can now see what it was impossible to detect from 

 a surface view, namely, that the wrinkles are compound. 



The larger, principal ones have secondary, smaller ones extending 

 outward from their sides, approximately at right angles. Schultze 

 observed and figured these compound wrinkles in his surface views of 

 R. temporaria, and adds another detail which I have been unable to 

 find in the Chorophilus eggs, viz., the breaking-up of a single wrinkle 

 at its peripheral end into several smaller ones arranged radially from a 

 common point. 



The pigment usually fills the projecting protoplasm between adjacent 

 sinuses. It is also much thicker in the region of the wrinkles than else- 

 where along the furrow, as can be seen in fig. 40. 



In view of these different facts, therefore, it seems evident that there 

 is an intimate relation between the wrinkles and the pigment — and that 

 both may be results of the same cause. It remains to ascertain what 

 this cause is, if possible. 



According to Schultze the egg is a single cell, and just as cellular 

 division is brought about by the contractility of protoplasm, so is the 

 segmentation of the egg due to the same cause. These contractions 

 originate at the point where the furrow begins, and are at first confined 

 to a very small area. Since the cortical portion of the egg protoplasm 

 possesses a glutinous consistency, it is not to be wondered at that folds 

 or wrinkles appear at the same time with the furrow, in its immediate 

 vicinity, and at right angles to it. These subsequently disappear in 

 consequence of the difference in contractility between the outer and 

 inner protoplasm, due to their different consistency. 



It is exceedingly difficult to understand how compound wrinkles, of 

 such a nature as we have just described, could be produced by the 

 simple contraction of a viscous cortical layer of protoplasm, especially 

 if that contraction starts from a fixed point in the layer. Indeed, how 



