CHAPTEE XXIV. 



TARDIGRADA: 



The eggs of the Tarcligrada are laid either singly {Macrobiotus 

 Hufelandi) or several together, and are left in the cast-off skin of 

 the mother. In the case of eggs laid singly, the egg-integument is 

 thickly studded with small prominences which render its examination 

 very difficult. When several eggs are laid together the egg-envelope 

 is smooth and transparent. The species investigated by Kaufmann 

 seems to have been comparatively easy to study, and he was able to 

 establish the fact that its cleavage is total and equal, as v. Siebold 

 had already stated. Kaufmann followed the process of cleavage up 

 to the formation of a morula stage composed of cells of about equal 

 size. He then observed the separation of a peripheral cell-layer from 

 the central mass, and the flexion of the embryo which supervenes. 

 The concave side of the embryo seems to correspond to the ventral 

 surface, for the limb-rudiments here arise on the two sides. Two 

 pairs of prominences appear first ; these are apparently the two 

 anterior pairs of limbs, which are followed by the third and fourth 

 pair. When the young leave the egg, they possess the full number 

 of limbs and have the general form of the mother. 



v. Siebold's statement (No. 4, p. 553), that the Emydiac have only three 

 pairs of limbs when they leave the egg, may be traced to a misunderstanding of 

 Doyere's account (No. 1). This author states that the limbs are here not fully 

 developed, not that one pair is wanting. It does not appear from v. Siebold's 

 account that he himself investigated this point, which is of interest in con- 

 nection with the comparison that has repeatedly been made between the 

 Tardigrada and the Acarina. 



The accounts of the ontogeny of the Tardigrada are unfortunately 

 so scanty that we can hardly gain anything from them applicable to 

 the whole group. We cannot even tell for certain if a blastoderm and 

 germ-band develop, although this is probable. The armature of the 

 mouth is evidently a product of the stomodaeum, as may be gathered 

 from the study of the adult anatomy ; mouth-parts (in the sense in 

 which the term is used of the Arthropoda) apparently do not appear 



