24 SALPA. 



In succeeding stages the tail gradually atrophies, and the gill slits, four 

 in number, develop ; at the same time a process or stolon, destined to 

 give rise by budding to a second non-sexual generation, makes its appear- 

 ance on the dorsal side in the seventh inter-muscular space. This stolon is 

 comparable with that which appears in the embryo of Salpa. When the 

 tail completely atrophies the larva leaves its transparent covering, and 

 becomes an asexual Doliolum with a dorsal stolon. 



SalpidcB. As is well known the chains of Salpa alone are sexual, and 

 from each individual of the chain only a single embryo is produced. The 

 ovum from which this embryo takes its origin is visible long before the 

 separate Salps of the chain have become completely developed. It is en- 

 veloped in a capsule continuous with a duct, which opens into the atrial 

 cavity, and is usually spoken of as the oviduct. The capsule with the 

 ovum is enveloped in a maternal blood sinus. Embryonic development 

 commences after the chain has become broken up, and the spermatozoa 

 derived from another individual would seem to be introduced to the ovum 

 through the oviduct. 



At the commencement of embryonic development the oviduct and ovi- 

 capsule undergo peculiar changes ; and in part at least give rise to a 

 structure subservient to the nutrition of the embryo, known as the placenta. 

 These changes commence with the shortening of the oviduct, and the dis- 

 appearance of a distinction between oviduct and ovicapsule. The cells 

 lining the innermost end of the capsule, i.e. that at the side of the ovum 

 turned away from the atrial cavity, become at the same time very columnar. 

 The part of the oviduct between the ovum and the atrial cavity dilates 

 into a sack, communicating on the one hand with the atrial cavity, and 

 on the other by a very narrow opening with the chamber in which the egg 

 is contained. This sack next becomes a prominence in the atrial cavity, 

 and eventually constitutes a brood-pouch. The prominence it forms is 

 covered by the lining of the atrial cavity, immediately within which is the 

 true wall of the sack. The external opening of the sack becomes gradually 

 narrowed and finally disappears. In the meantime the chamber in which 

 the embryo is at first placed acquires a larger and larger opening into the 

 sack; till finally the two chambers unite, and a single brood-pouch contain- 

 ing the embryo is thus produced. The inner wall of the chamber is 

 formed by the columnar cells already spoken of. They form the rudiment 

 of the placenta. The double wall of the outer part of the brood-pouch 

 becomes stretched by the growth of the embryo ; the inner of its two layers 

 then atrophies. The outer layer subsequently gives way, and becomes rolled 

 back so as to lie at the inner end of the embryo, leaving the latter projecting 

 freely into the atrial cavity. 



While these changes are taking place the placenta becomes fully 

 developed. The first rudiment of it consists, according to Salensky, of the 

 thickened cells of the ovicapsule only, though this view is dissented from 

 by Brooks, Todaro, etc. Its cells soon divide to form a largish mass, 

 which becomes attached to a part of the epiblast of the embryo. 



On the formation of the body cavity of the embryo a central 

 axial portion of the placenta becomes separated from a peripheral layer; 

 and a channel is left between them which leads from a maternal blood 

 sinus into the embryonic body cavity. The peripheral layer of the placenta 

 is formed of cells continuous with the epiblast of the embryo; while the 



