446 



INVERTEBRATA 



CHAP. 



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

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



en 



d a 



stom 



end 



slam 



FIG. 350. Median sagittal sections through three eml>ryos of Ascaris megalocephala, iii 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 tin- stomodaeal cells form a tube, and in which the mother cells of the genital organs are 

 covered in by ectoderm, hip, blastopore ; end, endoderm ; stout, stoinodaeuin. 



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

 rearrangement takes place in some of the ectoderm cells covering 



