48 KECORD OF CURRENT RESEARCHES RELATING TO 



been formed on the dorsal surface of the egg ; then, towards the 

 hinder end of these ridges, and between them, there appears a small 

 depression, which becomes converted into a funnel-shaped invagina- 

 tion ; this is the prostoma or anus of Rusconi, Observation of 

 median sections shows that the invagination presses towards the 

 central yolk-cells, and also reveals the presence of a primitive enteric 

 cavity ; in later stages a communication is to be observed between 

 the invaginated sac and the primitive enteron, which is effected by 

 means of a narrow cleft. When we compare this process with what 

 obtains in Petromyzon we find that the essential difference lies in the 

 greater width of the prostoma. "What we know of the mode of 

 formation of the gastrula in the Ganoids leads us to see a marked 

 resemblance between them and the Salamanders ; and we may sum up 

 what happens in saying that in all these three forms the invagination 

 is directed towards the yolk-cells which fill up the interior of the 

 egg; with these yolk-cells, the cells of the invaginated endoderm 

 become connected, and, with them, they bound the primitive enteric 

 tube ; this last and the neural tube communicate for a time by means 

 of the prostoma. 



In the Teleostei the relations are not quite the same ; but what 

 it is of importance to note is that the blastoderm does undergo an 

 invagination which has just the same relations as in the already 

 mentioned Vertebrata (in the middle line of the caudal end of the 

 embryo), and that this invagination gives rise to an epithelial sac ; 

 the latter was regarded by Kuppfer as the allantois, and he now sees 

 that it represents the primitive endoderm, although it is never, as in 

 the Eeptilia, invested by a mesodermal layer ; it is not converted 

 into the enteric epithelium, for this is derived from a set of cells 

 which appear to arise by free-cell-formation and which may be 

 known as the secondary endoderm; of this we may distinguish two 

 series, one belonging to the enteron and one to the epithelium of the 

 yolk-sac. Availing himself of Mr. Balfour's results, the author comes 

 to the conclusion that in the Elasmobranch fishes the method of 

 development is more similar to that which is seen in the Ganoids and 

 Batrachia than to what obtains in the Teleostei ; the cells of the 

 secondary endoderm, although they form the greater part of the 

 investment of the yolk-sac, do not form the whole of it. 



With regard to Mr. Balfour's observations on the Lacertilia* 

 Kuppfer thinks the difference in their results is chiefly due to the 

 condition of the English observer's specimens, and he believes that 

 the neuronteric canal of Balfour is his myelo-allautoid canal; it 

 may be noted that the presence of this canal has been observed by 

 Kuppfer in a chick three days old, where it is about 1 mm. long, 

 and opens into the dorsal medulla by an extremely narrow slit 

 (J^ mm.). Twenty-seven years ago Bischoff observed in the guinea- 

 pig the presence of a process, which beginning from the margin of 

 the embryonic shield grew inwards and downwards, and which, 

 altogether similar to the same development in the Reptilian egg, was 

 the earliest rudiment of the allantois. 



* 'Quart. Joiirn. Micr. Fci.,' xix. (1879). 



