1 1 4 WHEELER. [Vol. V 1 1 1 . 



and indistinct and disappears very early. Each of the succeed- 

 ing segments, with the exception of the eleventh abdominal 

 has a distinct pair of somites. In the last abdominal, meso- 

 derm is present, but I have been unable to find a trace of 

 a true ccelomic cavity, ^ 



The youngest embryo in which I was able to detect repro- 

 ductive cells had almost reached Stage F. In still earlier 

 stages careful scrutiny failed to reveal any differentiation of 

 the mesoderm cells. These cells, it is true, vary considerably 

 in size and appearance but I have found it impossible to fix 

 on any one set of elements which might be brought into con- 

 nection with the germ-cells of older embryos. It is not, 

 therefore, till the somites are established as distinct sacs 

 that unmistakable primitive germ-cells make their appearance. 

 In frontal sections of embryos in Stage F (Fig. ^2 gd^, gdz), 

 the primitive germ-cells are seen to lie in the inner wall of the 

 somite. They are considerably larger than any of the sur- 

 rounding mesoderm cells, and much paler. The chromatin of 

 their nuclei is arranged in a more delicate skein. Like the 

 neuroblasts they stain very deeply in picric acid. Normally, 

 they occur only in the first to the sixth abdominal segments, 

 each cluster being confined to the inner wall of a somite. The 

 reproductive organs of XipJiidium are therefore truly meta- 

 meric in their origin. There is nothing to show that they 

 arise from vitellophags which have migrated into the somitic 

 wall ; nor can they arise from the entoderm, since they are 

 differentiated before the entoderm-bands have reached the 

 basal abdominal segments in their growth backward from the 

 oral and forward from the anal formative centre. I conclude, 

 therefore, that the primitive germ-cells are enlarged and modi- 

 fied mesoderm-cells. In explanation of Fig. 52 it may be 

 noted that the plane of section is somewhat oblique so that it 



1 I mention this because Graber ('90) has recently described a ccelomic cavity 

 in the anal segment of Hydrophibis (Fig. 29, p. 62). Cholodkowsky also describes 

 and figures ('91, Fig. 49 PI. IV) such a cavity in the eleventh abdominal seg- 

 ment of Blatta. Every little slit in the mesoderm is not a ccelomic cavity, and 

 the figures referred to show only small spaces between the mesoderm cells of 

 the telson. This may have been produced artificially, for aught the figures show 

 to the contrary. 



