TERIPATUS NOVAE-ZEALANDIAE. 



167 



of the cells and their constant shifting towards the periphery, leads 

 to the almost complete circumcrescence of the yolk as far as a point 

 lying almost opposite the original accumulation of nuclei, where the 

 yolk remains uncovered. Here an ingrowth of cells then takes place, 

 the appearance of an invagination being thus produced (Fig. 77 A 

 and B). The point of invagination is the blastopore (bl), and the 

 base of the depression is formed of yolk in which nuclei can be 

 recognised (Fig. 77 B). The germ-layers do not yet appear to be 

 differentiated from the cell-mass surrounding the blastopore, which 

 represents the rudiment of the germ-band. Miss Sheldon seems to 

 assume that the part of the cell-mass underlying the superficial cell- 

 layer (or ectoderm) yields chiefly mesoderm, while the entoderm 

 arises from the cells lying in the yolk, and which, according to Miss 



Fig. 76. — Portions of sections through the egg of P. novae-zealandiae, showing the blastoderm- 

 formation (after L. Sheldon). A shows the " polar area " and the cleavage of the yolk. B, 

 the commencement of the circumcrescence of the egg. a, " polar area " ; ds, "yolk segments." 



Sheldon, arise and multiply by a process of free nuclear forma- 

 tion (I), as she was unable to observe any karyokinesis.* The 

 blastopore lengthens later and then resembles a small groove, the 

 base of which is formed by the nucleated yolk. We here have a 

 resemblance to the condition in P. capentis illustrated in Fig. 84 A. 



As far as we can gather from the description of L. Sheldon, the process of 

 circumcrescence is regarded by her as an epibolic gastrulation. The yolk-mass, 

 with the nuclei contained in it, would correspond to the entoderm. A study of 

 the figures, however, has compelled us to form another conclusion, which gains in 

 probability from the fact that we are here dealing, as in the case of many 



* [It is extremely doubtful if there is such a process as free nuclear formation. 

 All recent research on nuclei tends to prove that every nucleus originates from a 

 pre-existing nucleus either by mitotic or amitotic division. — Ed.] 



