ox THE FORMATION OF T]IE GERMINAL LAYERS IX PETROMYZOX. 133 



easily rcc()_i:^ni/.ed in the.se parts, especiaiJy on the floor, as they are so 

 niarkedlv diHeivni Irom the f'ells of the vegetative pole. Any trace of 

 sncli miiii-al ion is liowever not to Ije found. I am strono-]y inclined to be- 

 lieve that the t^liinning out: i.s !)ronght ah(Hit by the cells of different layers 

 in tlie roof oftiie .-egnietitation cavity wedging themselves in between 

 one anotlier, tliiis gradually rt^lucing the nundjer of layers. As this 

 |)rocev> of wedging in ne^'essarily increases the siiperiicial extension 

 the cells are a])le to spread themselves \o the invaginated tube as well 

 as over the cells of the \egerati\o ])o]e ))y epibole during the gastrula- 

 tion. rien''e the thinninu' out is continued and carried on at the time 

 that the invagination is ta.king ])lace. .Vccording to this view, the 

 definitive epiblast is not formed necessarily by the cells which formed 

 tlie outermost layer of the animal ])o!e in earlier stages, but by those 

 which were left after other cells have been invaginated. 



The ga.4rulation now c-ijurmeru'es. l*^gg-^ undergoing this process 



s1io\v several inarked clianges even wlien viewed as opaque objects. 



In the first place they ac(piire the ap|)erirance of being solid and firm, 



no di)ul)t in consequence of the obhteration of the segmentation cavity. 



Thev also decrease in their absolute size, as can be easily perceived by 



tlie naked eye. This nnist necessarily be the case when we remember 



that a large portion of the cells is in\agiiiated into the inside of the 



eo'i»". Tlie shape of the e"'o- also undero-oes chano-e. Instead of bein£>- al- 

 oe 1 an o o Ü3 



most Spherical a.> hithertof(_)re, they have become more or less elongated 

 in the longitudinal axis of the future embryo. The anterior end can 

 now be told from the posterior, as the former is rounded, while the 

 latter is pointed. The dorsal surface can likewise l)e distinguished 

 from tlie ventral, as the former shows a faint median ridge. Thus the 

 egg assumes a distinct bilateral symmetry. 



The internal changes accompanying these alterations in outward 



