yelk, and about fifteen or twenty egg-germs in earlier stages. The 

 development of the eggs reminded me much of that which takes place 

 in the Orthoptera. 



It appears to he a pretty well established fact that, at least in many 

 animals, the Purkinjean vesicle is a modified ovarian cell, round which 

 the yolk is deposited. On the other hand, according to some eminent 

 naturalists, there are certain animals in which the ovarian cell becomes 

 the egg, and its nucleus the Purkinjean vesicle. In this case we 

 should have two distinct classes of eggs, the Purkinjean vesicle in one 

 of which would be homologous with the whole egg in the other. I 

 have, however, given in my paper the reasons which induce me to doubt 

 whether these last observations are altogether correct. But even if 

 we may admit that no essential difference has as yet been proved to 

 exist in the eggs of animals, as far as regards the relations existing 

 between the Purkinjean vesicle and the original ovarian cell, it would 

 still seem that in the relations between the former and the yelk, two 

 very different types of development must be recognized. 



In describing the so-called "winter-ova" of Lacinularia socialis, 

 Prof. Huxley says (Micr. Journal, vol. i. p. 14), "It will be observed 

 that all these authors consider the winter-ova or ephippial ova and the 

 ordinary ova to be essentially identical, only that the former have an 

 outer case. The truth is that they are essentially different structures. 

 The true ova are single cells which have undergone a special develop- 

 ment. The ephippial ova are aggregations of cells (in fact, larger or 

 smaller portions, sometimes the whole of the ovary), which become 

 enveloped in a shell, and simulate true ova." This aggregation of 

 several cells (one of them putting on the appearance and fulfilling the 

 functions of a Purkinjean vesicle), and the whole becoming enveloped 

 in a shell, is, however, the ordinary and only method of egg-develop- 

 ment in many lower animals. In the Trematoda and Cestoid worms, 

 and the greater number of the Turbellaria, the yolk and the Purkin- 

 jean vesicle are formed in two separate organs. 



In Piscicola, according to Leydig, the mature egg contains, besides 

 the Purkinjean vesicle and the ordinary yolk, a number of nucleated 

 cells (Zeit. f. Wiss. Zool. 1849. part 1. pi. 10. fig. 56). 



In the Mites and Spiders, in Chelifer, Obisium, the Phalangidse, and, 

 so far as I know, all the Arachnida, the egg arises from the meta- 

 morphosis of a single cell. 



