238 EICHAED ASSHETON. 



The RabbiTj the Mole, the Shrew. 



The growth of an outer layer of cells round an inner core 

 of cells of rather different texture was in the first instance 

 described by van Beneden, 1875, which process he regarded as 

 a kind of gastrulation, and to the form produced thereby he 

 gave the name of metagastrula. The outer layer he called 

 epiblastj the inner core hypoblast. Van Beneden's description 

 is very complete, and was at first accepted, and his figures 

 passed into all text-books. 



From the researches of Rauber and KoUiker, however, it 

 was seen that this interpretation could not be maintained, 

 because, as they showed, and as was confirmed by van Beneden 

 also in a later paper (van Beneden and Julin, 14) , the 

 greater part of the inner core is transformed into the epiblast 

 of the actual embryo, while only a small portion becomes 

 hypoblast. But although the interpretation of the meta- 

 gastrula as given by van Beneden was thus shown to be in- 

 correct, does it necessitate the total abandonment of the 

 description given by van Beneden ? Does a "metagastrula" 

 stage not exist at all ? Duval, on finding in his bats an 

 identical process to that described by van Beneden for the 

 rabbit, re-adopts the metagastrula for both rabbit and bat. 



He writes (p. 153) : " Cette gastrulation est celle, sauf una 

 difference a preciser plus loin, que van Beneden a decrite 

 chez le lapin sous le nom de Metagastrula, et qu'il a ensuite 

 admise pour les Cheiropteres, d'apres I'etude d'une vesicule 

 blastodermique du Rhinolophe, vesicule formee d'une sphere 

 externe d'ectoderme et d'une mass interne d'endoderme. Or, 

 depuis ses deux meraoires de 1880, van Beneden a abandonne 

 sa conception de la metagastrula des mammiferes. Nous devons 

 declarer purement et simplement que nous reprenons cette 

 conception, et que nous nous pr^parons k la defendre." 



The evidence in favour of the occurrence of the metagastrula 

 in the rabbit may be briefly tabulated. 



(1) Foremost is van Beueden's (8, 9) original account. The 



