804 DEVELOPMENT. [CH. LVIII. 



contains a granular material, vitelline or yolk substance which 

 nourishes the protoplasmic part. The way in which ova are 

 formed in the ovary is described in the chapter preceding this. 



In many animals, as in birds, most of the changes from ovum to 

 adult take place, not within the body of the parent, but outside of 

 it. The egg can therefore derive no nutriment from the mother ; 

 and, as one would expect, the yolk substance is much more 

 abundant. It is due to this fact that the eggs of birds, reptiles, 

 and fishes are so much larger than the mammalian ovum. Each, 

 however, is still essentially a single cell, much bulged by yolk 

 material. The yellow part of the hen's egg which is alone com- 

 parable to the mammalian ovum has upon it a whitish speck or 

 cicatricula about inch in diameter ; in this the nucleus or 

 germinal vesicle is imbedded ; it is here and in the surrounding 

 protoplasm that cell division and growth goes on, the rest of the 

 yolk serving to nourish this part. 



Ova in which the whole cell takes part in cell division are 

 called holoblastic ; but those like the hen's egg, in which only a 

 part, the cicatricula just alluded to, divides and subdivides, are 

 termed meroblastic. There are, however, gradations between the 

 two extremes. 



The structure of the mammalian ovum has already been given 

 (p. 801). The surrounding zona pellucida is perforated, at any 

 rate in some animals, by a small hole called the micropyle, which 

 enables a spermatozoon to enter. Some observers have described 

 a second more delicate membrane within the zona pellucida. 



Changes in the Ovum previous to Fecundation. 



The most important change is the disappearance of the greater 

 part of the nucleus ; simultaneously with this two particles called 

 the polar or directing globules appear on the surface of the ovum, 

 between it and the zona pellucida. 



The relation between these two phenomena, and many of the 

 facts relating to the impregnation of, and early changes in the 

 ovum were first made out by Oscar Hertwig, and Ed. v. Beneden, 

 in the ova of sea-urchins and other echinoderms ; they have 

 been verified in many of the higher animals ; and, though they 

 have never been actually seen in the human ovum, there is 

 practically no doubt that they occur there too. 



In the case of the sea-urchin, the problem is a comparatively 

 easy one to study. Sea-water containing the elements of each 

 sex can be obtained, the changes watched, and finally the two 

 specimens can be mixed and impregnation observed. 



In the ovum, the nucleus travels to the surface of the ovum, 



