ON Tin: (JASTJUr.ATlON IN PKTUOMYZON. 2/ 



alouc [/'if/s. IG-LS, /Ndc. cj).) ; it is probably tliis i)iiit ui' thu 

 epitlieliuni, wliicli misled Goette and Lwoff to assume the 

 arcbeuteric root' as beiug, in its entire extent, formed of an 

 irregular epitlieliuni 1-2 cells in thickness. As soon as the limbs 

 of the arc commence to come nearer each other {Fig. 8c), the 

 microraeric epithelium appears inside in the archeuteric roof 

 {Fig. l^O ; it is thus supposable that the approximation of the 

 arc-limbs, /. e., of the lateral blastoporic lips, stands in an 

 intimate connection with the contribution of the micromeric 

 epithelium to the formation of the archenteric roof, as the follow- 

 ing consideration w'ill show. 



With the commencing infolding of the micromeric epithel- 

 ium, the blastoporic lip is strongly raised in its median part 

 {Fig. Sc), and the slit-like passage of the blastopore is converted 

 at this part into a circular pore. This circumstance is brought 

 aljout hy the complete withdrawal within of the macromeres in 

 this part and by the subsequent lifting of the blastoporic lip 

 formed of the micromeric epithelium which is now lieing in- 

 vaginated. For illustration of this point, the three figures. 

 Figs. 27-29, are instructive ; they were drawn from a series of 

 sections through an ovum a little younger than that shown in 

 Fig. 8a or Fig. 19 and represent cross-sections passing respectively 

 across the ventral part {Fig. 27), the blastoporic passage {Fig' 

 28) and the dorsal li}) of the blastopore {Fig. 29). In the first 

 figure {Fig. 27), the micromeric layer {e. riiic. ep.) is not yet 

 turned inw^ards, though the invagination of the macromeres has 

 proceeded to a great extent {hp.). In the next figure {Fig. 28), 

 the macromeric field is no longer exposed on the external surface 

 of the ovum, but has retired far inside; and the blastoporic passage 

 is bordered, from right and left, by the lateral walls formed of 



