No. 2.] THE OVARIAN EGG OF LIMULUS. 167 



It occupies the meshes of the cytoreticulum, and appears to be 

 movable from one point to another, according to the condition 

 of the controlling living substance. It is, therefore, subject 

 to changes in mass, giving considerable variation to the appear- 

 ance of the cytoplasm. It must be pointed out here that the 

 unequal distribution of the yolk, as well as the variable conden- 

 sation of the living substance in different parts of the egg, 

 which is frequently to be observed, does not alter the spherical 

 form of the egg. It is hardly probable, therefore, that the 

 spherical form is due to surface tension, which implies an 

 equilibrium of similar molecules in all radii. 



In the younger eggs the amount of yolk varies considerably. 

 It may be so abundant as to obscure the cytoreticulum, or it 

 may be very limited in amount. It has been shown that the 

 earliest formed eggs grow more rapidly than those formed in 

 the period preceding sexual maturity. In the latter the yolk 

 is sometimes relatively scarce, and the cytoreticulum is very 

 distinct. In such cases the nutriment of the egg is presumably 

 so limited that the surplus food material is used up in the 

 growth of the living substance. At any rate, it is certain that 

 the egg increases in size by the growth of the living substance, 

 and by mechanical expansion due to the accumulation of yolk. 

 The growth of the cytoreticulum predominates in the earlier 

 stages, while the accumulation of yolk is the chief cause of 

 increase in size after the eggs are discharged from the follicle. 



It has been maintained that the yolk originates in all cases 

 within the egg, and it appears to be with considerable reluc- 

 tance that many, even now, admit the origin of yolk in any 

 other way. This reluctance seems to date back to the early 

 controversy regarding the cell nature of the egg. On the one 

 side it was claimed that the yolk spheres represent real cells ; 

 on the other, that the yolk originates within the egg. 



An external origin of the yolk has frequently been main- 

 tained. I need only mention Ayers ('84), confirmed by L. 

 Will ('84), in the egg of Oecanthus niveus. 



There are reasons for believing that in Limulus a substance 

 having the essential characteristics of yolk is produced in 

 the epithelial cells ; and that this, in the form of granules, 



