38 K. MITSUKUW AND 0. ISHIKAWA. 



through the epiblast to the space below the hypoblast.^' Here 

 Balfour does not say how this passage arises. In his ' Com- 

 parative Embryology' (vol. ii, p. 168) he says: "At the front 

 end of the primitive streak an epiblastic involution appears, 

 which soon becomes extended into a passage open at both 

 extremities, leading obliquely forwards through the epiblast to 

 the space below the hypoblast." Kupffer (No. 5) is of sub- 

 stantially the same view. Weldon (No. 14, p. 136) says : " At 

 a point {bp.), however, the position of the future blastopore, 

 these layers are replaced by a mass of closely-packed cells 

 {pr.), exhibiting no division into layers, and forming the 

 primitive streak, which may, in some cases at least, extend 

 backwards as far as the commencement of the area opaca. The 

 blastopore commences at the anterior end of this streak as a 



pit, open above and closed below The floor of this pit 



presently breaks up, and the blastopore assumes its normal 

 condition, forming a communication between the archenteron 

 and the exterior, its anterior wall forming a communication 

 between the epiblast and the lower layer cells. From this time 

 a change in the character of the lower layer cells takes place, 

 beginning from the anterior wall of the blastopore, where they 

 pass into the epiblast, and proceeding forwards. Instead of 

 being large, irregular, full of yolk, as in the previous stages, 

 they become columnar, lose their yolk, arrange themselves in 

 a definite layer several cells deep, and take on the characters 

 of normal hypoblast This process is evidently an in- 

 vagination comparable to that which takes place in an Elasmo- 

 branch. It especially resembles the process described by 

 Scott and Osborne in the newt." Strahl gave his views first 

 in an article published in 1882 (No. 8), and again in a later 

 writing (No. 13, p. 55) . His views, as expressed in the latter, 

 are briefly as follows : — Before the neurenteric canal is present 

 the germinal disc consists throughout only of ectoblast and 

 entoblast, except in the region of the primitive streak, which 

 is oval or pear-shaped, or nearly triangular in form. In such 

 a disc three processes, which may be independent of one 

 another, now take place. 



