532 DEVELOPMENT OF THE SIMPLE TISSUES, BY S. STRICKER. 



acid, and the thick vitelline membrane be cautiously peeled off, 

 the different stages of the process of investment may be fol- 

 lowed with the naked eye. Before the cap covers the first 

 third of the yolk sphere, it may be seen that the thickened 

 border is particularly enlarged at one part. With a lens the 

 dorsal groove may here be seen, which is directed towards the 

 superior pole (the original position of the germ). 



As the cap enlarges, this rudiment of the embryo increases in 

 size, forming a thickened cord or column of the cap extending 

 from the thickened border towards the upper .pole. Finally, 

 when the border is reduced to a slender ring, scarcely visible 

 to the naked eye, it bears a relation to the embryo (the thick- 

 ened cord) which recalls in a lively manner the relation in 

 which Rusconi's anal aperture of the Batrachian ovum stands 

 to the axis of the dorsal half. Rusconi has himself called 

 attention to this resemblance. 



In the Batrachian ovum the dorsal furrow extends from the 

 anal aperture towards the superior pole, though it does not 

 quite reach to it. The thickened cord representing the dorsum 

 of the embryo of the Forella has precisely the same relation 

 to the annular remains of the thickened border which, as is 

 self-evident, bounds a canal. In the egg of the Forella, how- 

 ever, the yolk is exposed by means of this canal, whilst in the 

 Batrachian ovum, in which there is no yolk, large cleavage cells 

 are exposed. 



The whole remainder of the cap constitutes to some extent 

 the wall of the body, but is in chief part a yolk sac.* The centre 

 of the germ disk, which has thus been shown originally to lie 

 over the germ cavity, is in Fowls the most important part, 

 being in fact the proper embryo, whilst in Fishes it forms only 

 the rudiment of the yolk sac. In both germs the deep-lying 

 large cells fall upon the floor of the germ cavity. In Fowls, 

 however, a layer remains behind to form the intestinal glan- 

 dular layer. In the Forella all fall down, and scarcely any 

 glandular lamina is formed in the centre. 



If now, at the time when the attenuated centre (S) of the 



* See my illustrations in the Sitzungsberichte der Wiener Akademie, 

 Band li. 



