DEVELOPMENT OF PHEYGANIDS. 591 



and in many cases we must look beyond this for an adequate 

 explanation of the facts. 



The yolk doubtless plays a very important part in modifying 

 the segmentation of the egg ; the amount is so great that if it 

 were intimately combined with the protoplasm it would prove a 

 very serious obstacle to segmentation, and hence to accelerate 

 development it is from the first entirely separated from it. 



We may conceive that the germinal vesicle is surrounded 

 by an irregular mass of protoplasm, scattered widely through 

 the yolk. When the nucleus divides, only a small portion of the 

 protoplasm collects around the resulting nuclei, the remaining 

 portion being irregularly distributed throughout the yolk. 

 After a number of germinal cells have been formed, exactly 

 the same conditions would exist that we have first observed 

 (PI. XXXVlB, fig. 1). 



Both the germ-cells and outlying protoplasm are subjected to 

 some centrifugal influence which propels them towards the sur- 

 face. This might give rise to difi'erent results, varying with the 

 amount and condition of the yolk. It would be a very slight 

 and unimportant modification of the more ordinary course of 

 events to have the network of protoplasm reach the periphery 

 first, and thus form a superficial protoplasmic layer before the 

 less mobile nuclei could migrate through the yolk and reach the 

 surface. The impediment offered to the nuclei by the yolk 

 would easily account for the presence in some insects (Musca, 

 Chironomus, &c.) of what Weismann named the Keimhaut- 

 blastem, whereas the absence or diminution of this impediment 

 would in other cases (e. g. Aphis, &c.) allow the nuclei to arrive 

 at the surface simultaneously with the accumulation of proto- 

 plasm. The Phryganids offer a condition which is intermediate 

 between these extremes. 



One of the most remarkable results due to the presence of so 

 much yolk is the production of three distinct stages in the seg- 

 mentation of the egg; first, the germinal vesicle with its 

 protoplasm divides and produces a number of cells which unite 

 at the surface to form a syncytium; second, the syncytium 

 quickly divides into a number of cells with cell walls ; and third. 



