ox THE GASTRULATION IN PETROMYZON. 7 



Calberla^^ and A. Goette.-^ Gocttc regarded the eminence as the 

 equivalent of the " Kandwulst " of the Teleostean ovum and of 

 the " Randzone " of the Amphibian ovum, while it seems not to 

 have been understood by the two other authors at any rate. To 

 a discussion on this point, I will return further on. 



Let us now turn to follow the further fate of the two de- 

 pressions above described, by which way the superficial changes 

 of the gastrulation can be best learned. 



In the first place, the history of the blastoporic depression 

 will be dealt with. The limbs of the dorsal lip which overhangs 

 the depression extend themselves further and ventrad along a 

 level somewhat below the boundary groove above referred to, 

 describing a large arc {MffS. la and 7c, bp.). The part of 

 the ridge thus added likewise overhangs the depression at every 

 part, but it becomes gradually lower towards both its distal ex- 

 tremities, finally to become lost on the general surface of the ovum. 



This addition of the blastoporic \\\) in its extent is, however, 

 brought about not by lateral prolongation of the blastoporic 

 depression in its slit-like form, as might be supposed, but is 

 produced by the opaque hemisphere generally sinking down in 

 its proximal largest part. To do this, a small field of the 

 hemisphere which lies immediately behind (ventral to) the slit 

 is at first flattened {Fig. 6c, bii.) ; then, while the flattening is 

 further extended into the ventral part, the flattened area is gradually 

 changed into a large shallow depression which is deeper towards 

 the median and the steepest part of the dorsal lip {Fig. Ic, bp). 

 In other words, the slit-like blastopore is now converted into the 



1) E. Calberla: Zur Enlwickelung des Medullarrohrs und der Chorda dorsalis der 

 Teleostier und Pelroniyzonten. Morphol. Jabrb., Bd. 3, 1877. 



2) A. Goette: Entwickelungsgeschichte des Flu&sneuuauges (Petromyzon fliiviatilis). 

 Hamburg und Leipzig. 1890. 



