18 The Microscope. 



and which exercises important influences over the early stages 

 of the egg, is " food-yolk " or deutoplasm, as it is usually 

 termed. The function of this is to supply nutriment to the 

 growing embryo, and by the vital forces of the egg it is gradu- 

 ally transformed into protoplasm as it is needed. The proto- 

 plasm of the egg is clear, homogeneous and transparent, but 

 the deutoplasm is usually broken up into granules, so that it is 

 nearly or quite opaque. This is not always the case, for in many 

 pelagic fish-eggs the food-yolk is as clear and transparent as the 

 protoplasm. The food-yolk is very variously distributed. In 

 some it may occur in equal amounts in all parts of the egg, or 

 it may be concentrated at the centre, or at one of the poles, or 

 intermediate conditions may exist; and each varying arrange- 

 ment is the cause of great differences in the development of 

 the egg. The protoplasm is the active, the deutoplasm the 

 passive element. 



a. Maturition. 

 After the egg is fully grown there remains one last step in 

 the ripening process, which is known as the maturition of the 

 ovum. Some of the points of this ripening have long been 

 known, but their significance was first pointed out by Minot 

 and Balfour some five years ago, almost simultaneously. To 

 study these phenomena at all carefully one needs a small and 

 nearly transparent egg, for the successive stages can best be 

 studied in the living ovum, and for this reason the most detailed 

 accounts we have relate to the eggs of the star fishes and sea- 

 urchins. At the time that the egg leaves the ovary if possesses 

 the essential parts enumerated above, but shortly begins to 

 undergo some of the most wonderful changes. In the case of 

 the star-fish eggs they begin as the egg enters the salt water. 

 These changes are most apparent in the nucleus. First the 

 wall of the nucleus breaks down and the clear protoplasm 

 contained in it takes an irregular outline, the nucleolus 



Fig. 1. — Portion of ojrg with the nucleus and nucleolus broken down; 



u, the lust remnants. 



