20 K. MITSUKURI; 



have chosen for illustrating the course of the changes. Did it not 

 increase figures to an inconvenient number, many other embryos 

 might be represented, which would show phases of development similar 

 to, or intermediate between, the stages given. 



Fig. 2 has already been referred to, as showing the condition 

 when the posterior portions of the medullary folds have pressed 

 towards the median line and reduced the yolk-plug to a wedge-like 

 shape, by embracing it between their posterior ends. Now follows a 

 series of changes which, if expressed simply from the appearances 

 observable on the surface and without any regard to the internal 

 processes giving rise to them, may be objectively described as 

 follows : — 



The icedge-shaped yolk-pliig, from this stage on, gradually shifts its 

 position backwards. In doing so, it generally leaves in its track a shallow 

 median groove ivhich might be mistaken for the continuation of the medullary 

 canal, although, it can be easily distinguished from the latter by being 

 narrower, shallower, and often more or less wavy. 



I may state here by anticipation that I consider this groove as 

 the primitive groove, and the region between the posterior portion of the 

 medullary canal (with the blastoporic passage now changed into the 

 neurenteric canal) and the backward- shifted yolk-plug as the region 

 of the primitive streak. The cause of this backward shifting of the 

 yolk-plug we must consider to be the extremely active growth 

 and multiplication of the cells lying in the posterior portion of the 

 embryo. These statements will be better understood, when the 

 sections h.'ive been described. The backward progress of the yolk- 

 plug at any given stage may roughly be estimated by its distance 

 from the neurenteric canal and its position in regard to the " End- 

 wulst " (terminal knob). 



In Fig. 3 the medullary canal is already closed, except at its 



