234 K. MITSUKURI AND C. ISHIKAWA. 



Th<i formation of the Blastoporlc Passage. 



There are difference« of opinion among previous writers on the 

 subject in regard to the formation of the blastoporic passage in 

 Reptilia. Balfour (No. 2, p. 424-5) says : " After the segmentation 

 and the formation of the embryonic shield (area pellucida) the blasto- 

 derm becomes distinctly divided into epiblast and hypoblast. At the 

 hind end of tlie shield a somewhat triangular primitive streak is 

 formed by the fusion of the epiblast and hypoblast, with a number 

 of cells between them, which are probably derived from the lower 

 rows of the segmentation cells. At the front end of the streak a 

 passage arises, open at both extremities, leading obliquely forwards 

 through the epiblast to the space below the hypoblast." Here Bal- 

 four does not say how this passage arises. In his ' Comparative 

 Embryology' (vol. ii, ]). 168) he says: "At the front end of the 

 primitive streak an epiblastic involution appears, which soon becomes 

 extended into a passnge open at both extremities, leading obliquely 

 forwards through the epiblast to the space below the hypoblast." 

 Kupffer (No. 5) is of substantially the same view. Weldon (No. 14, 

 p. 136) says: "At a point (&/>.), however, the position of the future 

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

 (j)!'.), exhibiting no division into layers, and forming the primitive 

 streak, Avhich may, in some cases at least, extend backwards as flir 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 normabconditon, forming a communication between the 

 archenteron and the exterior, its anterior wall forming a communica- 

 tion l)etween the epiblast and the lower layer cells. From this time 

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



