76 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



shown by the fact that in Clavelina, Ciona, and Styelopsis the 

 doubling of the germinative epithelium occurs before the oogonia are 

 formed. 



III. Incubatory Pouch. 



1. Historical Summary. 



Almost all compound and social Ascidians have some provision for 

 retaining their eggs within the colony until the larvae developed from 

 them are ready to enter upon their free swimming existence. The 

 organ where this is accomplished is usually called an incubatory pouch, 

 but its structure varies much in different groups. Usually the pouch 

 is merely a slight enlargement of one peribranchial sac, into the 

 bottom of which the eggs are laid, and from whose top the fully formed 

 larvae escape. Occasionally, however, as in Glossophorum sabulosum 

 described by Lahille ('90, p. 203), the pouch consists in the terminal 

 enlargement of the oviduct. Distaplia and Colella, and probably also 

 the allied genus Julinia, Caiman ('94), present an extreme develop- 

 ment of incubatory pouch, which consists of a large diverticulum con- 

 necting with the dorsal edge of the zooid by a narrow stalk. Within 

 the enlarged terminal part of the pouch the eggs and embryos are 

 always arranged so that the youngest are at the bottom; but how they 

 got there has been somewhat of a puzzle to ascidiologists. Delle Yalle 

 ('81) published the first account of this structure; he supposed that the 

 ripe ova reached the peribranchial sac and were put into the pouch 

 together, and then fertilized. The retardation that the spermatozoa 

 would experience in travelling down the pouch and getting past the 

 first ova he thought accounted for the greater development of the 

 embryos near the mouth of the pouch. The next investigator of the 

 subject was Herdman ('86) , who studied this organ in Colella pedun- 

 culata and allied species. He describes the pouch (p. 89) as " merely 

 an enormous diverticulum of the peribranchial or atrial cavity " and 

 remarks that the neck is so narrow that ova can pass in but that the 

 larvae cannot escape. He explains the arrangement of the embryos 

 within the pouch by supposing that the ripe eggs all reach the peri- 

 branchial sac, are there fertilized and subsequently put into the pouch 

 in order, the one last fertilized going first. But he says that he has 

 no evidence for this, as eggs have never been found in the peribranchial 

 sac. Since then no investigator, so far as I know, has attempted to 

 account for the arrangement of embryos in incubatory pouchas of the 



