584 SUMMARY OF CURRENT RESEARCHES RELATING TO 



and syncytial nature of the embryo, as to which he has already made some 

 remarks. Peripatus capensis is remarkable, even if not unique, among 

 animals for the large size of its egg, combined with the almost complete 

 absence of yolk ; the history of its segmentation shows that at no period of 

 development are tbe cells which arise from that segmentation completely 

 isolated units ; on the other hand it is quite certain that in small holoblastic 

 eggs the cleavage is complete, and the question naturally arises, is the com- 

 plete or the incomplete cleavage phylogenetically the more correct. In 

 answer to this we may observe that no animal is composed of a mass of 

 separate and similar cells, that complete cleavage is probably very much 

 rarer than is generally supposed ; when such a cleavage does take place it 

 may possibly be due to "an intensely active force in the centre of the celi, 

 which compels for the moment the assumption of this (clean rounded) form 

 in the protoplasm over which it has dominion." Tbe phenomena of seg- 

 mentation in the ova of various Crustacea suggest that it may be possible 

 to find a purely mechanical explanation of complete cleavage. The sup- 

 position that the ancestral Metazoon was a colonial Protozoon is not 

 supported by holoblastic segmentation, but is somewhat favoured by what 

 we know about incomplete cleavage. 



The view, however, which is in accordance with the facts of develop- 

 ment of Peripatus capensis is the old doctrine that the ancestral Metazoon 

 was a multinucleated infusorian-like animal. But this view is, after all, 

 only " a more or less plausible suggestion without any strong basis in fact." 

 Mr. Sedgwick proceeds to criticize the speculations of Metschnikofif, and 

 points out certain difficulties and misunderstandings ; the chief point in 

 which they disagree is that Mr. Sedgwick cannot accept the view that the 

 hollow blastula is a primitive form, or that the formation of the endoderm 

 by migration inwards of the cells is a primary process. Attention is 

 directed to the fact that the formation of mesoderm in Peripatus is 

 essentially a formation of nuclei which pass to their respective positions 

 and arrange themselves in the protoplasmic reticulum there present, and to 

 the observation that the primitive streak is the growing point of the animal, 

 from which almost all the tissues of the adult are derived; its nuclei, 

 therefore, are not merely mesodermal, but are also ectodermal and 

 endodermal. 



S. Araclmida. 



Morphological Significance of so-called Malpighian Vessels of two 

 Spiders.* — Dr. J. C. C. Loman has made transverse sections through the 

 hinder part of the body of a Cteniza from West Java, the examination of 

 which shows that the two excretory ducts are appendages of the midgut ; 

 similar relations have been observed in Epeira, Tegenaria, and Mygale. 

 Other points of difference between these ducts and the Malpighian vessels 

 of insects are to be found in (1) the structure of the separate cells, which 

 in spiders are of the type of enteric epithelial cells, and (2) as to their 

 function, for the contents of the spider's ducts are fluid, and their slight 

 contents are by no means of the character of renal concretions ; uric acid 

 and uric salts were wanting. 



The resemblance of these tubes to the tubular excretory organs which 

 have been shown by Spencer to be connected with the midgut in Oniscus, 

 Gammarus, &c., is another indication of that connection between the 

 Arachnida and the Crustacea rather than the Insecta, which recent studies 

 have gone so far to support. 



* Tijdschr. Nedcrl. Dierk. Vcrecii., i. (188G-7) pp. 109-13. 



