90 K. MITSUKURI; 



the nulime.ntani part referred to hj Balfour in the above qvotation from 

 the ' Comparative Emhriiolo<iii.^ " 



The itiilics of the last few lines are mine. In the scheme of 

 homology which I h;ive sketched out ahove ( pp. 83~S5), the part, e f, 

 in Sedgwick's woodcut ( = Woodcut XII of the present article) 

 which is stated hy him to he rudimentary or ahsent in Aves and 

 Amniota generally, is distinctly present in Chelonia as a part of the 

 primitive streak. The fact is that neither Balfour nor Sedgwick 

 in making the ahove comparison ever thought that there was a 

 rudimentary yolk-structure in the Aminota which ought to he com- 

 pared with the yolk-blastopore of Elasmohranchii, and directlv homo- 

 logised the large yolk-mass of the Sauropsida with the yolk-mnss 

 of Elasmohranchii. This view I consider to he untenable, as will be 

 evident from what is said under the next headincf. 



The identification of the primitive streak with the blastopore is 

 no new idea, but I hope to have filled in details which make the 

 comparison much more accurate. 



2. The problematical cell-mass which stands up between the 

 blastopore lips, and which I have all along called the yolk-pluf/, 

 represents, in such a chelonian embryo as that of Figs. S and S«, the 

 large yolk-mass of the Elasmobranch <à<>:<r. Flence, we nuist conclude 

 that tlie main hodif of tlte i/olk in tJie chelonian eg<j must not he homo- 

 logised iritlt the i/olk-mass of tlie Elasmohranch egf). It seems most 

 reasonable to conclude that in course of phijlogenetic development, the ijolk 

 u'liicli is Itomologous irith tlie golk-mass of the Elasmohranch egg, mvst have 

 dwindled in size and Jteen lost from the eggs of the ancestors of tlie 

 Amniota. This yolk-mass ivhich 1 mag call the primary yolk vtass 

 has, however, left in some eggs, {e. g., in the chelonian egg) its rndiments 

 in the structure known as the p)rivntire plate and its direct derivative, 

 the problematical cell mass behind tlie blastopore. In further course of 



