UROCHORDA. 29 



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 appearance 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. 



SalpidaB. 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 

 enveloped 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 com- 

 mences 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 

 ovicapsule 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 

 disappearance 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 containing 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. 



