50 ART. 3. S. IKEDA : CONTRIBUTIONS 



Verwertung des durch die Furchung geschaffenen Zellenmaterials 

 beim Aufbau des Embryos in erster Linie abhängt von der Zellen- 

 umlagerungen, welche bei der Gastrulation von sich gehen, so 

 müssen wir die Vorgänge bei der Gastrulation zum Mittelpunkte 

 der Betrachtung machen," (p. 19). 



6). In Rhaœphorus eggs, the gradual growth of the neural 

 plate is indicated by a very conspicuous structural change. It is 

 that the lower or neural layer of the epiblast becomes two-cell 

 layered. This has been already noticed by Assheton ('94a) who 

 says : " In the frog the nervous layer soon becomes thickened " 

 (the number of cell-layers not specified) " along the future dorsal 

 surface of the embryo, and over the rest of the embryo the nervous 

 layer becomes reduced to a layer of one cell only in thickness, like 

 the epidermic layer " (p. 169). This process takes place not only 

 on the dorsal blastopore lip (Fig. 65 a) but also all around the rim 

 of the blastopore (Fig. 65 b). However, no further progress is made 

 except at the dorsal median portion where it spreads gradually 

 forward and laterally over the dorsal surface of the future embryo 

 (Fig. 66 1J ), until it reaches a point 30 c -40° distant from the 

 original upper pole when it stops. The region over which this 

 change takes place I have indicated in Figs. 54-60 by crossed 

 lines and in Woodcuts I-IV., by heavy black lines. By a careful 

 study, I have ascertained that the upper limit of the two-cell 

 layered portion of the mucous layer coincides approximately with 

 the forward extension of the archenteric cavity at every stage, and 

 therefore at the final stage marks the head end of the future 

 embryo. This again shows clearly that the location of the embryo 

 in the egg is along the meridian of the blastopore with its head 

 end situated 30°-40° from the original upper pole of the egg. 



1) Dorsal portion of the egg in Fig. 57 under a higher power. 



