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 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 2 inches long at birth 

 in a Salpa {S. Forskalii) a foot 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 Salpa embryo 

 from a minute tgg, 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 Salpa 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 Salp)a 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. 



