ORIGIN OF THE MESODERMAL STRUCTURES. 209 



which develops the genital aperture (in P. capensia this is the segment 

 of the anal papillae, and in the American species the antepenultimate 

 segment).* An actual separation into a lateral and a dorso-median 

 section, such as takes place in the other primitive segments, does 

 not, however, occur in the genital segment ; here, indeed, the 

 primitive somite extends dorsally, but its widened dorsal portion 

 remains connected with the lateral portion. After this primitive 

 segment, like the rudiments of the nephridia, has acquired an 

 external aperture, its dorsal part fuses with the posterior ends of 

 each of the tubular genital glands, and the rudiment of the genital 

 organs is thus essentially completed. The two external apertures 

 shift towards the middle line so as to lie beside one another. An 

 invagination of the ectoderm then yields the unpaired terminal 

 region (ductus ejaculatorius, vagina) of the genital apparatus. 



The development of the genital organs makes it evident that the cavity of 

 the genital glands is homologous with the secondary body-cavity for coelom). 

 Its cellular lining thus corresponds to the peritoneal epithelium of the Annelida ; 

 in both cases the genital products become detached from this, fall into the 

 secondary body-cavity, i.e., in Peripcdus the cavity of the genital glands, and 

 pass out of the body through the nephridia. That the efferent ducts of the 

 genital apparatus in Peripatus are homologous with the nephridia cannot be 

 doubted. This is not only proved by their manner of developing, but is also 

 confirmed by the fact that in the American species the nephridia are wanting 

 in the antepenultimate segment, which carries the genital apertures, while they 

 are regularly developed in the preceding segments and in the segment that 

 follows (Gaffron). 



The transformation of nephridia into efferent genital ducts such as are found 

 in Peripatus is of special interest, because of the continuity of the transformed 

 nephridia with the genital glands, and because this demonstrates a similar 

 morphological constitution of the whole of the genital apparatus, such as is 

 met with in other Arthropoda. This continuity tends to mask the nature of 

 the efferent ducts ; their true character as nephridia can only be ascertained 

 with certainty from the study of their development. 



Another account of the origin of the Mesodermal Structures. 



The description given by v. Kexxel of the transformation of the primitive 

 segments differs in some essential points from those of the English authors. 

 Since those points are of great importance, they must be separately discussed. 



According to v. Kexxel, besides the cell-growth which, in the form of a 

 fold projecting from below, divides the segmental cavity into two spaces (Fig. 

 100 £, p. 200), a second fold grows in from the inner wall of the cavity 

 (Fig. 101 A), so that the cavity is divided into three spaces which for a time 

 are in communication (Fig. 10-1 A, I, II, III). The dorso-median portion (III) 

 then becomes partitioned off, and this as well as the greater part of the lateral 

 portion, which lies principally in the foot, is used up in providing elements 



* Cf. on this point the remarks on the anal glands, p. 207. 



P 



