W. K. BROOKS ON THE GENUS SALPA. 247 



supporting ring is very simple in this species, where its function is to 

 give support, like a pessary, to the embryo at its point of attachment to 

 the wall of the atrium, and by its rigidity to maintain a free channel for 

 the circulation of the blood through the narrow neck of the placenta into 

 and out of its cavity. This appears to be its primary function in all the 

 species, as Barrois points out in the case of Salpa africana. He says, 

 p. 495. its chief function is to support the embryo and its membranes, 

 upright in the middle of the uterine cavity (cavity of embryo sac). In 

 this species the inner ends of its cells soon become dotted by a row of 

 circular spots which are shown in Figs. 3 and 4. These spots are so much 

 like the muscle fibers seen in sections of the tissues of coelenterates, as to 

 suggest the thought that they may form a sphincter of epithelio-muscular 

 fibers around the neck of the placenta. 



I have had no fresh specimens for teazing, and my series of embryos 

 is so scanty that I have not had the material for further investigation of 

 this point, although it seems of sufficient interest to deserve the attention 

 of histologists. 



SECTION 4. The Embryo Sac. 



It is well known that in most species of salpa the embryo becomes 

 inclosed in a brood chamber or incubatory pouch which is formed about 

 it by a circular collar-like fold in the wall of the cloaca, around the area 

 where it is attached. 



While the fold is not formed out of the tissues of the embryo, like 

 the amnion of insects and the higher vertebrates, its mode of origin and 

 its anatomical relations to the embryo are so similar that the word 

 "amnion" comes naturally to the mind, although the term embryo sac 

 seems better. 



Around the neck of the placenta the epithelial capsule is at first 

 continuous with the ordinary epithelium of the atrium, as shown in 

 Plate XI, Fig. ], but soon a circular collar-like fold, Plate XLV, Fig. 2, 

 21 and 22, is formed around the area of attachment. This fold is the 

 amnion, or according to Salensky terminology, the " Faltenhulle." 



It never attains to any considerable size in Salpa hexagona, and 

 Salensky states that it is absent in a very similar species, Salpa demo- 

 cratica. It is shown in three successive stages in Salpa hexagona, at 21 

 and 22, in Plate XLV, Figs. 2, 3 and 4. 



In Salpa pinnata, at a very early stage, the amniotic fold grows up 

 on all sides of the embryo until this is completely shut in to a brood 



