446 



INVERTEBEATA 



CHAP. 



by the gradual approximation of its sides, and it is for a brief period 

 long and slit-like (Figs. 350, 351). 



end 



FIG. 350. Median sagittal sections through three embryos of Ascaris megalocephcda, in order 

 to show the invagination of the genital cells and of the stomodaeal rudiment. (After Boveri.) 

 A, embryo at the close of gastrulation ; a small part of the endoderm is still uncovered. B, 

 embryo in which gastrulation is complete ; the stomodaeal cells begin to be invaginated. C, embryo 

 in which the stomodaeal cells form a tube, and in which the mother cells of the genital organs are 

 covered in by ectoderm, blp, blastopore ; end, endoderm ; stom, stomodaeum. 



As the tail cells multiply and the embryo elongates, a remarkable 

 rearrangement takes place in some of the ectoderm cells covering 



