ox THE GASTRULATION IN PETROMYZON. 9 



else than the ventrally prolonged arms of the arched lips which 

 had heretofore bounded the pore on the doisal and lateral sides : 

 the limbs have now become united at the median point of the 

 ventral junction-line. In this way, the hhistopore is for the 

 first time surrounded all around by a lip. It should, however, 

 be specially mentioned that the lip is in the ventral part very 

 slight in elevation so that the general surface of the ovum still 

 passes over into the floor of the invaginated pocket at a wide 

 angle. 



Now the gastrulation is completed ; no trace of the opaque 

 area can be detected on the external surface of the ovum. The 

 ovum is much reduced in bulk, so that it is in size about two- 

 thirds of that of the stage of Fig. 2 and shows a solid, instead of 

 a hollow, consistence. 



It is a remarkable fact that the part of the opaque area 

 which answers to the yolk-plug in the Amphibian ova is not 

 represented here at any stage, as will be seen from the above 

 accounts. A yolk-plug is present only when the invagination of 

 the yolk field is delayed to a late stage in which the ventral lip 

 of the blastopore is completely established, as is best seen in the 

 Amphibian ova. 



The process mentioned above was observed by Max 8chultze, 

 A. Shipley and A. Goette. Shipley states: "The invagination 

 at first has a wide-arched slit-like opening, but this soon narrows 

 into a small circular pore "'\ but he says nothing further on this 

 important part of the history ; the sinking of the opaque hemi- 

 sphere in the shape of a hollow funnel did not, it seems to me, 

 attract his attention. Max Schnitze"^ knew some of the changes 



1) A. Shipley : On some Points in the Development of Petromyzon fluviatilis. Quart. 

 Journ. Microic. Sc., 1887, p. 5. 



2) loc. cit. p. 13. 



