W. K. BROOKS ON THE GENUS SALPA. 49 



embryo is bathed by the water which is constantly flowing past it, and it 

 is therefore in very much closer relation to the external world than a 

 mammalian embryo, shut up in the interior of a large thick-walled 

 body. There does not seem to be any need in salpa for a respiratory 

 placenta, and its thick spongy walls seem to indicate that it is not respir- 

 atory. We find in its structure nothing like the interlacing villi of the 

 mammalian chorion, and the sections show that the embryo is nourished 

 in a way quite unlike anything which has been described in the mam- 

 malia. 



The subject is a very interesting one. The rapid growth of the salpa 

 embryo is one of its most conspicuous characteristics, and the nutrition 

 which this rapid growth demands is secured by two very peculiar organs, 

 the follicle and the placenta. 



While the egg at the time of fertilization is very minute, the embryo 

 at the time of birth is enormous, as compared with the size of the chain- 

 salpa which carries it, and it certainly increases many thousandfold 

 during development. The growth is only partially due to cell multipli- 

 cation, and it is in part a result of the growth of the individual cells, for 

 instead of growing smaller with repeated division, they actually increase 

 in size in all parts of the body. 



As the older stages are less magnified in the figures than the younger 

 ones, this growth of the cells is not conspicuous in the figures, but it is 

 one of the most notable peculiarities of the salpa embryo, and in many 

 parts of its body cells as large as the original ovum are found. The 

 growth sets in very early, and it goes on uninterruptedly throughout the 

 whole foetal life, so that the embryo becomes gigantic as compared with 

 the body of the chain-salpa which contains it. Quoy and Gaimard 

 describe an embryo, two inches long at birth, in a salpa (S. forskalii) a 

 foot long, and Leuckart says that the embryo of S. democrat] ca at birth 

 is two-fifths as long as the chain-salpa which carries it. The fully grown 

 embryo of S. hexagona is almost as long in comparison with the chain 

 form of the same species. 



It is not unusual for the embryos of viviparous animals to gain 

 slightly in size and weight before birth, but, as Leuckart points out, the 

 mammals are the only animals which exhibit anything comparable to 

 the rapid growth of the salpa embryo from a minute egg, and the history 

 of the salpa embryo at once calls to mind the growth of the embryo in 

 the placental mammals; nor is this resemblance entirely superficial, for 

 in both the mammal and in salpa we find an especial foetal organ, the 



