90 JOURNAL OF THE 



not, owing to the limited time over which my observations have 

 as yet extended, earefnlly determined the progress of develop- 

 ment. Figs. 8 and 9 [)rohably represent the stage where the 

 larger endoderra (internal) cells are completely surnninded by 

 the smaller ectoderm (external) cells, just prior to the invagina- 

 tion (sinking in) of the head end to form the mouth and (eso])h- 

 agns. 



AccordiDs: to Strubell,* in Helerodera Schachtii, the first two unequal cells into 

 which the egg divides represent primary' cells of two diiterent groups of cells 

 which result from farther division. The larger prioaary cell divides more rapidly 

 and forms small cells, which grow around the more slowly formed lar^rer cells 

 Avhich result from the division of the other smaller primary cell. The growing 

 over proceeds first down the convex side of the egg and the ectoderm cells fold 

 over the opposite end of the embryo, the mass of endoderm cells. Thus the 

 " prostom " (the open space between the converging edges of Ihe enveloping 

 ectoderm cells) is on the concave side of the e^g, and bv^^cause 'he ectoderm cells 

 on the concave side of the head end have grown but little it (the prostom) occu- 

 pies the entire concave (ventral) side of the young embryo. At this stage if we 

 turn the e^s, so that we are looking directly at the concave side the ectoderm 

 cells will be in a boat-shaped mass, and in this boat-shaped mass of ectoderm 

 cells will be the larger endoderm cells. The " prostom " (open part of the boat- 

 shaped mass of ectoderm cells) now begins to close by the growth and increase 

 of the cells at the margin. This closure takes place more rapidly at the posterior 

 end and advances toward the head end, so that after awhile there is only a small 

 opening through the ectoderm cells near the head end of the concave side. This 

 is finally closed so that the endoderm cells are completely enveloped by the 

 smaller ectoderm cells. 



This is probably the staoi:e which I have fiii^ured in Fig's. 8 and 

 9, Plate IV. The larger, endoderm, cells can be seen in the 

 centre; the smaller, ectoderm, cells on the outside. Invagina- 

 tion of the ectoderm cells now takes phu-e at the head end, that 

 is, the cells sink inward as if pushed in by some outside force. 

 This is represented in Figs. %x and 10. By this process the 

 mouth and cesophagus are developed. I have only studied the 

 external changes in the embryonic development. From this 

 point up to the fully developed larva the changes are represented 

 in Figs. 11 to 15. Beginning yith Figs. 10 and 11 the head id\\(\ 

 appears hyaline and tinely granular, and is larger in diameter 

 than the rest of the young embryo, which at this stage is of 



*Untersucliungen uber den Ban und die Entwickeliuig des Rubennematoden, Hete- 

 rodera Schachtii Schmidt. Hibliotheca zoologioa, Hel't 2, 1888. 



