370 Mr. W. K. Brooks on the 
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-multiplication, 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. 
This growth of the cells is one of the most notable pecu- 
liarities of the Sal/pa 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 2 inches long at birth 
in a Salpa (S. Forskaliz) a toot long, and Leuckart says that 
the embryo of S. democratica 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 Sa/pa embryo 
from a minute egg, and the history of the Salpa embryo at 
once calls to mind that of the placental mammals ; nor is this 
resemblance entirely superficial, for in both the mammal and 
in Salpa we find an especial foetal organ, the placenta, for 
the purpose of affording to the growing embryo an abundant 
supply of nutriment. 
The resemblance between the foetal life of Sa/pa and that 
of a mammal is most remarkable, and it is all the more note- 
worthy since we may be absolutely confident that the placenta 
of Salpa is an independent acquisition, entirely without 
genetic relation to that of mammals. 
No modern writer except Todarro has ventured to regard 
the two structures as homologous, and their phylogenetic 
independence is so obvious that it is not necessary to discuss 
it, although a greater physiological and anatomical resem- 
blance than the facts warrant has usually been assumed. 
We should hardly expect fundamental similarity in struc- 
tures of diverse origin. On the contrary, we might reasonably 
look for profound differences between the placenta of Salpa 
and that of the mammals. 
