Gastrulation in the Pigeon's Egg. 85 



and laterally reduces the entire central region of the blastodisc to a 

 single layered epithelium. Concerning this extension of the dif- 

 ferentiation, he writes as follows : "Die epitheliale Schichte ist jetzt 

 in ziemlich gleichmassiger Weise an der ganzen Oberflache des 

 Blastoderms zur Ausbildung gekommen (Fig. 2). Offenbar sind 

 also die Zellen, welche in dem friiheren Stadium (Fig. 1) den 

 dickeren Theil des Blastoderms bildeten, in dieses epitheliale Blatt 

 eingetreten, indem die tieferen Zellen sich aufwarts nach der 

 Peripherie bewegten und sich dem Epithel einordneten ; daher nahm 

 die epitheliale Schicht betrachtlich an Ausdehnung zu und in Folge 

 dessen hat das Blastoderm jetzt eine grossere Liinge und Breite und 

 ist ein Umstiilpungsvorgang am Hinterrande des Blastoderms einge- 

 leitet worden."^^ 



Almost the same words could be employed in describing the 

 changes which occur in a pigeon's blastoderm after it has reached a 

 stage corresponding to that shown in Fig. VI. Hence, the process 

 of thinning-out of the avian blastoderm, as well as that of the 

 selachian, is to be regarded as homologous with the process of blastula- 

 tion in Amphioxus and Petromyzon. There will be, undoubtedly, 

 a "wide difference of opinion as to the advisability of using the term 

 "blastulation" to describe this process ; for the term blastula has 

 been employed for stages which cover a wide period of development. 

 Ordinarily, however, it is used to designate that stage of development 

 just preceding the gastrula-invagination — a stage in which the seg- 

 mentation cavity is more or less enlarged. The result is that the 

 so-called blastulse of the various vertebrates have not the same 

 morphological value. 



As to when one should call the pigeon's egg a blastula, will depend 

 on the criteria adopted. Using the term as it is variously applied 

 among the different vertebrates, one might speak of a blastula from 

 the eight-cell stage to the beginning of invagination, and, adopting 

 Hatta's suggestion, even to the end of gastrulation. It is obvious, 

 therefore, that the term could be used only in the most general 

 way. I prefer to avoid it altogether, and for that reason, shall 



^^Loc. cit., p. 58, 



