ox Tili: GASTllLLATIOX IN PETROMYZOX. Ö 



{Fig- 2), (lie long axis of which pusses throiigli from pole to 

 pole. The superficial area of the traiisluceiit hemisphere has been 

 greatly added to since the last stage, while the opaque part has 

 still further diminished in extent ; it now occupies the lower 

 fifth of the ovum, as will be seen from Fig. 2. To simplify descrip- 

 tion, the translucent hemisphere may be called the upper, and 

 the opaque the lower, hemisphere. 



This peculiar stage has been, so far as I am aware, entirely 

 overlooked by all the previous authors. Max Schnitze is the only 

 author^^ who figured correctly {Fig. 15, Taf. I.) the ovum of the 

 stage in question, but he seems to have paid no special attention 

 to this peculiar shape of the ovum and gives no account of it. 

 The same omission was committed by the present author ; Fig. 

 1, PL I, given by him in the above cited paper shows an oblique 

 section through a little further advanced ovum in which the 

 gastrula invagination is about to take place. 



Now at a certain point in the line of junction"^ of the trans- 

 lucent hemisphere with the opaque, a shallow depression trans- 

 verse to the long axis appears {Figs. 3« and 3^, bg.) ; this gradual- 

 ly extends itself to the right and left along the junction line, 

 and at the same time it deepens, so that the depression, or as it 

 may now be called the groove, is deepest in the part that appeared 

 first and is shallowed out towards both its extremities. The 

 deepest part in the middle represents, as further history will 

 show, the dorsal side of the forthcoming creature. 



The groove in question is not difficult to make out ; the 

 chorion which in the foregoing stages had been closely applied 



1) Max Schultze : Die Entwickelungsgeschichte von Petromyzon Planeri. Haarlem, 

 1856. 



2) This line will be called, in the following pages, the junction-line, denoting simply 

 the boundary between the translucent and opaiiue parts of the ovum. 



