28 COMPARATIVE ANATOMY chap. 



C. The quantity of formative yolk is, in comparison with the 

 enormously developed nutritive yolk, so small that it is only a small 

 mass enclosing the germinal vesicle at the animal pole, and a very 

 thin layer round the whole egg. In by far the greater part of the 

 egg the formative yolk is quite supplanted by the nutritive yolk 

 (meroblastie teloleeithal eggs, Fig. 23, E). 



II. Types of Centroleeithal Eggs. 



The formative yolk is arranged in a regular layer round the whole 

 egg, and besides this in a mass containing the germinal vesicle at the 

 centre of the egg. The region between the centre and the circum- 

 ference is occupied by nutritive yolk, either 



A. In largely preponderating quantity (holoblastie centroleeithal 

 eggs of many Crustaceans), or 



B. Almost, or quite exclusively (meroblastie centroleeithal eggs of 

 the Tracheaia and many Crustaceans, Fig. 23, D). 



The Egg Envelopes. 



These are divided into (1) primary envelopes — yolk membranes, 

 egg membranes ; and (2) secondary accessory envelopes. 



I. The yolk membrane is secreted by the yolk itself. It can be 

 formed at different stages of the ripening of the egg, and shows great 

 diversity of structure. Occasionally it is double. It is often pene- 

 trated by numerous pores (zona radiata). Not infrequently there is 

 a special opening, the mieropyle. Both the pores and the micropyle 

 are connected with the nutrition of the egg, and serve in many cases 

 for the passage of the spermatozoa. 



II. The secondary envelopes are also very various. They are 

 alike in one point — none of them are formed by the egg itself. 



a. The chorion is a membrane which is very often secreted by 

 the cells of the ovary which surround the egg (follicle cells). It lies 

 between the egg and the follicle cells. 



b. Other accessory envelopes are sometimes found, which are 

 only added to the egg later by means of special glands on its way 

 through the passages leading out of the ovaries. Such are the egg 

 capsules, albuminous and gelatinous envelopes, calcareous shells, etc. 

 Either only one of these egg envelopes is formed, or else two or more 

 are simultaneously produced. 



As an example of the development and ripening of an egg, we choose first the 

 egg-formation of the mussel (Fig. 24, A, B, O). The eggs here arise out of definite 

 cells of the germinal epithelium, which grow more strongly and soon project beyond 

 the epithelium, with which, however, they remain for a time connected by a long 

 stalk-like process. Through this stalk, in all probability, the nourishment of the 

 egg by the epithelium takes place. Yolk granules, continually increasing in number, 

 appear in the protoplasm of the egg. The nucleus becomes vesicular. The egg 

 secretes on its surface ■■• yolk integument, which is broken through at the point 



