260 ^- MITSUKURI. 



between them, and such is actually the case as seen in Figs. 15 

 and 32 of Duval (No. 3). The annexed woodcut will also explain 

 why the posterior limit of the primitive streak is not as sharply 

 defined as the anterior. This view makes it plain that the 

 homologue of the primitive streak in Chelonia is the lips of the 

 blastopore which are, however, still so wide apart from each other 

 that the name " streak " is hardly applicable to it. It should be 

 noted that the primitive plate or knob is not the homologue of the 

 primitive streak. Tlie latter has potentially in it not only that but 

 a great deal more. It is in fact a mass of raw undifferentiated 

 material from which various structures are produced. This view also 

 makes it evident that as the |)rimitive streak is almost the first feature 

 visible in the development of the avian blastoderm, a great many 

 changes of palingenetic character observed in the chelonia n eg^ before 

 the establishment of what corresponds to the primitive streak, are 

 necessarily skipped over in Aves, which are therefore not very good 

 subjects in which to study the process of gastrulation. The removal 

 of the primitive streak to tlie centre of tlie blastoderm must also be 

 explained in the way I have indicated abovein the case of Chelonia. 

 Comparison of the reptilian ovum with the mammalian seems 

 easier. The facts given in this paper agree, with the exception of 

 some minor details, very closely with those communicated in \ an 

 Beneden's preliminary notice (No. 13). It seems to me that the 

 primitive streak and Heiîsen's knob together corresj^ond to the 

 primitive plate of Chelonia, and the " Kopffortsatz '" to the forward 

 growth of the primitive pJate. I can not, however, accept A an 

 Beneden's theory of '' Lecit()})hor " and "• Ijlasto])hor." Exactly what 

 I cannot accept lies in the emphasized words of the following qucjta- 

 tion: — " Wenn diese Auseinandersetzuno'en richtio* sind, "vvie ich es 

 glaube, so ist es klar dass das sogenannte zweiblätterige Stadium 



