410 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Tho endoderm in stage G becomes reduced to a layer of extremo 

 tenuity ; soon, however, it begins to increase in thickness; the endodcrmal 

 part of the alimentary canal is without glandular appendages of any kind. 



The mesoderm may be considered under four heads : — 



(1) The muscles of the skin arise from tho subectodermal fibrous 

 network, the outer part of which becomes arranged in a circular manner, 

 and so forms the circular muscles of the body-wall ; the longitudinal 

 muscles arise in seven patches. The contractilo tissue of the gut-wall 

 and internal organs generally is derived from wandering cells, which 

 themselves appear to be derived from the walls of the mesodermal 

 somites. 



(2) The body-cavity is vascular in type, and may, as Lankester 

 suggests, be called the " ha3mocoele." The heart, in stage G, becomes a 

 tube with thin walls and flattened nuclei, lying freely in the pericardial 

 cavity, with cellular cords projecting from its walls into the latter ; 

 these cords seem to become transformed into a pericardial network, 

 which contains round nuclei in its nodes, and is continuous with the 

 floor and roof of the pericardium. The hsemoccele becomes divided 

 into five main chambers — central compartment of the body-cavity, heart, 

 pericardial cavity, two lateral compartments in which the nerve-cords 

 and salivary glands lie, and the leg-cavities which contain the nephridia. 



(3) Mr. Sedgwick recapitulates the history of the nephridia under 

 the head of the somites from which they are respectively derived. 



(4) The generative organs, in stage G, form two tubes lying in the 

 central compartment of the body-cavity, and closely applied to one 

 another in the middle line. The generative ducts may be regarded as 

 modified nephridia. 



It is commonly said that, in the Arthropoda, the generative ducts 

 are continuous with the glands, but in Peripatus, at any rate, they 

 present exactly the same relation to the gonads as do the oviducts of 

 the dogfish or the earthworm to the ovaries of these animals ; or, in 

 other words, the generative ducts open into the ccelom, and the ova are 

 products of the ccelomic epithelium. 



Not only has Peripatus nephridia, but the coxal glands oiLimulus, and 

 the antennary glands of Crustacea are, as Lankester has suggested, pro- 

 bably nephridia ; so that the negative feature often regarded as charac- 

 teristic of Arthropods — the absence of nephridia — can no longer be 

 considered as justified by the facts of the case. 



Development of a South. American Peripatus.* — Mr. W. L. Sclater 

 has an account of the early stages of development of a South American 

 species of Peripatus from Demerara, which he proposes to call P. Im- 

 thurmi. The egg is small, as in P. torquatus and P. Edwardsii, and there 

 is an " extraordinary discrepancy " between its early stages and those of 

 P. capensis. The segmentation is complete, and there is no appearance 

 of sponginess, such as is described by Sedgwick for P. capensis, nor 

 would one suppose from the nature and size of the ovum that it had 

 only comparatively recently lost its yolk. The only similar case known 

 to us is that of placental mammals, and in both cases there appears to 

 have been diminution in the size of the ovum, total segmentation, and the 

 formation of an embryonic (Peripatus) or blastodermic (mammals) vesicle. 



A curious phenomenon, of which the author can offer no explanation, 



* Quart. Joum. Micr. Sci., xxviii. (1888) pp. 343-63 (1 pi.). 



