288 



K. MITSUKURI. 



never been treated with the fullness which its importance deserves. A 

 careful study of this process has given me results which, I venture to 

 think, are of the greatest importance in discussing the problem of 

 gastrulation in the secondary meroblastic egg. 



The surface views, Figs. 2-5, and the sections. Figs. 13-17 are 

 introduced to illustrate this process. Figs. 2 and 2a are of the 

 stage nearest to that represented in Fig. 1. In the dorsal view (Fig. 

 2), the dorsal opening of the invagination- cavity has now become a 

 narrow crescent-shaped slit with the concavity directed forwards.* 

 In the ventral view, the primitive knob has become larger. Viewed 

 with a low power, the surface of the knob is tolerably smooth, although 

 the figure represents it perhaps as a little too much so. The longi- 

 tudinal section (Fig. 18) of this embryo shows distinctly that the depth 

 of the primitive knob has grown greater in this stage than in that of 

 Fig. 9. The invagination-cavity has extended itself much deeper and 

 shows distinctly two limbs, one vertical and one horizontal. The roof 

 of the cavity which is as before continuous with the epiblast, shows 

 a distinctly columnar arrangement which is, however, gradually lost 

 both anteriorly and superiorly. In these directions it merges gradual- 

 ly into an irregular network of cells which is in turn continuous 

 with the lower layer of the embryonic shield. As was the case in 

 the former stage, there is again below the primitive knob, no in- 

 dependent sheet of cells continuous with the lower layer of the shield, 

 as described by Wenckebach or Mehnert. On the contrary, this and 

 the succeeding figures (Figs. 14-17) give the impression that the 

 lower layer of the embryonic shield extends below the epiblast 



* Will (No. 21. p. Ii7) says: " Dieselbe {i.e. die Urmundspalte) tritt zuerst im vorderen 

 Abschnitt der Primitivplatte auf, und hat zunächst die Form einer Sic hei- rinne, nach 

 Schwund der Sichelhörner aber einer rundlichen Delle" That is, his figures 8 and 9 are 

 less advanced than his figures 4 and 10 so far as the shape of the blastopore is concerned. If 

 the first two figures named are comparable to my figures 2, 3, 4, and the latter figures (his 

 figures 4 and 10) to my figure 1, I can not but think that Will is mistaken in his views. 



