ON THE FOETAL MEMBRANES OF CHELONIA. 37 



stages. Being possessed with the idea obtained fi-om the study of 

 Birds thnt it is soon to disappear, different writers have not thought 

 it worth theii- while to follow its history further. Nevertheless I ran 

 not but think that the sero-aniniotir- connection rmis a similar course 

 ill other groups of Re))tili;i as descril)e(| now for Ohelonia. I also 

 think that the posterior tube of the amnion is not such an unique 

 structure as it appears to V)e at present. 



The fifth point, the pi-esence of the rudimentary placenta, is 

 certainly very interesting. If the depression into which the white 

 is received shoidd Ijeconie deeper, ajid the allantoic folds -should be 

 produced to enclose it, we shall have exactly the same structure as 

 "the placenta " described by Duval in Birds. 



The Re])tilia, being the lowest group of the Amniota, are 

 of great importance in tlic (•<)niparative study of the foetal 

 membranes. What light dc^cs the history of the Chelouian fœtal 

 membranes as given aliove throw on the phvl<\f;"eny of those 

 membranes in the \'ei-febrat;i ? Witliout going into a o'enet'a.l disiiis- 

 sion of this difficidî prol)letu. I think T might offer here a few 

 suggestions which have |)f(-sented themselves to me in the course of 

 the present investigation. T strongly incline to the \ iew thnt the 

 amnion was originally descloped by mechanical cjuises. In Ohelonia, 

 when the head fold is |»ro(bicetl. there are two reasons why it should 

 sink into the yolk helow. fn the first place, the yolk is very Inro-c 

 and licpiid. especially just beneath the blastoderm, so that a slight 

 weight is enough to sink any struetiu'e into it. In the second place, 

 the white rapidly disappeai's from oNcr the blastoderm, which adheres 

 then firmly to the sh<'ll-niembrane : hence there is no space fo)* 

 Ihe head-fold to grow except towards l)elo\v. Without asserting 

 tluit these are the very same causes that produced the anterior 



