208 ONYCHOPHORA. 



known by L. Sheldon (No. 13) that, in P. novae-zealandiae, in the so-called 

 anal segment, there are two coiled glandular tubes, each of which opens 

 independently at the side of the body and laterally to the nerve-trunks, i.e., 

 at a spot where normally the nephridial apertures open. These two glands 

 are the equivalents of the anal glands (Sedgwick, Sheldon), and are more 

 correctly called accessory glands of the male genital apparatus ; from their 

 position, they may safely be regarded as modified nephridia. It should be 

 mentioned further that the American species, which thus shows the more 

 primitive condition in the segmentation of the posterior end of the body, shows 

 on the other hand a less primitive method of reproduction. The shifting of 

 the anus forward from the terminal segment must, indeed, in any case be 

 regarded as secondary. 



The Genital Organs. 



In the fifteen anterior segments of the embryo of P. capensis, the 

 dorso-median portions of the primitive segments are concerned in 

 the formation of the pericardium and heart, but in the following 

 segments their fate is quite different. After their separation from 

 the lateral or nephridial portions, they shift towards the dorsal 

 median line, and, decreasing in size, come to lie as small triangular 

 sacs between the wall of the intestine and the pericardium (Fig. 

 102, g). It is these, according to Sedgwick, which yield the 

 genital glands. Cells appear in them at a very early stage ; these, 

 which are distinguished by their size and specially large nuclei, 

 are the primitive genital cells. We might assume with v. Kennel, 

 that these arise in the wall of the primitive segments themselves, or 

 in the mesoderm-mass, before it breaks up into primitive segments, 

 as will be described later in connection with the Insecta. On the 

 other hand, Sedgwick ascribes an entodermal origin to the genital 

 cells. [See Editorial Preface, Vol. II.] 



By the fusion of the dorsal portions of the primitive somites 

 pertaining to consecutive body-segments and the breaking through 

 of their transverse walls, two tubes are formed, and these come 

 to lie in the middle division of the body-cavity. Up to this point, 

 the rudiments of the genital organs are alike in the two sexes, but 

 a histological differentiation now takes place, inasmuch as the genital 

 cells increase more rapidly in the male, and become smaller, whereas 

 in the female the germ-cells retain their large size. There is also an 

 anatomical differentiation, the genital rudiments in the female fusing 

 at the anterior end, while in the male they remain distinct in corre- 

 spondence with the form of the genital apparatus in the adult. 



We must assume that the median portions of these posterior 

 primitive segments yield the genital glands, while the efferent ducts 

 are derived from the lateral portions of that primitive segment 



