225^ 



the yolk to the centre of the (originally) lower surface. Also an 

 examination with the inverted microscope, of the perfectly normal 

 egg, leads to the conclusion that the dorsal lip travels at any rate 

 over a part of the white surface. 



Some observations on the normal develoi)men t. Be- 

 fore describing the pricking experiments, I wish to make brief 

 mention of some observations on eggs that were allowed to develop 

 under normal conditions. 



In eggs placed over a mirror and studied with a lens, the blasto- 

 pore when the lip first becomes differentiated all round as an edge, 

 occupies a central position on the under hemisphere, not an excentrie 

 position as in Pflüger's ('83) Figs. 23, 24. The circular or nearly 

 circular blastopore narrows greatly, continuing to occupy a central 

 position. Finally owing to a change in specific gravity of the different 

 parts of the egg (brought about by the enlargement of the archen- 

 teron, with concomitant suppression of the segmentation cavity), a 

 rotation of the whole egg through 90 ^ takes place — the direction 

 of rotation being such that the dorsal lip moves upwards, the blasto- 

 pore coming to occupy the position of a terminal of the horizontal 

 axis. My observations on all of these points agree with those of 

 ScHULTZE ('88). The excentrie position of the blastopore in the 

 eggs shown in Pelüger's Fig. 23 and 24, must I think have been 

 brought about by some change in the position of the whole egg. 

 Owing to the possibility, that is always present, of the occurrence of 

 some such change in the position of the egg, as a whole, it seems 

 to me that the apparent position of the blastopore (position in the 

 mirror picture of lower hemisphere) at successive times, is of little 

 value as evidence bearing on the question whether both lips move 

 across the yolk during gastrulation, or only the dorsal, or only the 

 ventral lip. 



The more conspicuous features in the process of delamination 

 and the outlining of the blastopore lip, described in a former section, 

 may very conveniently be observed, with the help of an inverted micro- 

 scope (using a Zeiss A or ^/^ inch objective), in an egg kept in the 

 normal position. For such observations the egg was placed in a 

 hollow slide, and without being compressed was covered with a cover 

 glass. The continuous observation was begun just before the appear- 

 ance of the dorsal lip; and a series of sketches at short intervals 

 was made for each egg. In eggs so placed, owing to the curvature, 

 it is more difficult to follow the history of individual cells in the 

 peripheral region, then in compressed and inverted eggs. In some 



Anat. Anz. XVIII. Aufsätze. 15 



