MUSEUM OF COMPARATIVE ZOOLOGY. 311 



is not at all divided, and in Fig. 1G it is divided into four large 

 spherules, enclosed in a common membrane. The food-yolk now 

 gradually becomes invaginated into the germ-yolk, as shown in Fig. 17, 

 and is soon entirely surrounded by the latter, forming the symmetri- 

 cal vase shaped "gastrula," shown in Fig. 18.* The cavity of the 

 gastrula opens directly into the sinus system of the nurse, and the 

 blood now circulates into and out of the primitive digestive cavity, as 

 well as around the outside of the embryo, which grows rapidly, and 

 soon fills the brood-sac, so that its outer surface comes into contact with 

 the latter, which soon ceases to be visible as a separate covering (Figs. 

 19 and 20), and of course the blood no longer bathes the outside of 

 the embryo, although it continues to pass into and out of the primi- 

 tive digestive cavity. The germ-yolk now becomes finely segmented, 

 although the spherules are still somewhat larger than the more trans- 

 parent ectoderm cells outside them. The invagination cavity or 

 "primitive digestive cavity" becomes separated into two portions; the 

 outer remains in free communication with the sinus system of the 

 nurse and forms the inner chamber (Fig. 19, 10) of the placenta, and 

 its opening becomes the orifice of the placenta. This persistence and 

 functional importance of the "orifice of Rusconi" are very remarkable, 

 and seem to have no parallel among the other Tunicata, or in any of 

 the various groups of animals with which it has been proposed to as- 

 sociate them. The gastrula of Salpa seems to be a special adaptation 

 to the very anomalous mode of development of the embryo. The lower 

 portion of the primitive digestive cavity now becomes entirely sur- 

 rounded by the endoderm, and soon becomes obliterated, as shown in 

 Fig. 18. Very soon a cavity reappears in this portion M the embryo, 

 and persists and forms the cavity of the branchial sac (Fig. 20, 13). A 

 constriction now appears upon the outside of the embryo, separating 

 the placenta from the embryo proper, and soon a body cavity becomes 

 visible, separating the branchial sac from the outer wall, and also ex- 

 tending up around the placenta, to form its outer chamber (Fig. 22, 12). 

 The placenta, therefore, consists of an inner chamber communicat- 

 ing with the sinus of the nurse, and having no communication with 



* A number of eggs at all stages between the two represented in Figs. 17 and 18 were 

 found, but the two here shown seem to be all that are necessary for clearly represent- 

 ing the process. Huxley (Plate XVI. Fig. 7) gives a figure of a stage in which the 

 invagination is about "naif completed, but he does not refer to ii in the text. 



