310 BULLETIN OF THE 



dow seems to be prolonged into its cavity, although at an earlier 

 stage the epithelium of the capsule appeared to surround the entire 

 surface of the yolk. The swelling and shortening go on rapidly, and 

 in a short time the egg presents the appearance shown in Fig. 12. A 

 single nucleus can now be seen at the point previously occupied by 

 the germinative vesicle, and the egg, nourished by the blood which 

 bathes it, begins to grow, and is already somewhat larger than before 

 impregnation. The shortening of the gubernaculum continues until 

 the egg is drawn down from the median neural surface of the nurse 

 to the point upon the right side of the lower or haemal surface of the 

 branchial sac, where the gubernaculum is joined to the latter, as 

 already described. The brood-sac, or cup-like depression of the 

 branchial sac, has meanwhile increased in size, and now forms a 

 nearly hemispherical cup, large enough to contain the egg, which is 

 soon entirely withdrawn into it, as shown in Figs. 13 to 18. Since 

 the cavity of the brood-sac is a diverticulum from the sinus system 

 of the nurse, the blood has free access to it, and bathes the egg on all 

 sides. The latter is perceptibly larger at this time than it was dur- 

 ing the stage last described, and this process of growth continues 

 during the whole of the subsequent development ; so that the 

 embryo, at the time when the first traces of organs make their 

 appearance, is many times larger .than the unimpregnated ovum, 

 and when the solitary embryo escapes from the body of the Chain- 

 salpa it is two or three times as large as the latter itself was at the 

 time when the egg was fertilized. This remarkable growth is men- 

 tioned here, in advance, as it will not be referred to in the subsequent 

 description, although it must be understood as going on at all stages. 



The egg is now pear-shaped, and is attached by its broad end to the 

 floor of the brood-sac, at the point where the gubernaculum origi- 

 nally joined the latter (see Fig. 13); the nucleus is now divided into 

 two, all traces of the epithelial capsule have disappeared, and nothing 

 more is known about it. 



Of the two nuclei now present the lower, with the portion of yolk 

 which surrounds it, is destined to form the " germ-yolk " of the em- 

 bryo, and soon divides again, as shown in Fig. 14, and at a stage a 

 little later, Fig. 15, it is composed of a mass of minute segments. 

 The upper nucleus of Fig. 13 with its portion of yolk forms the so- 

 called "food-yolk," and segments much more slowly. In Fig. 15 it 



