ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 27 



the secondary hypoblast ; the mesoblast in Petromyzon, just as in the 

 Sturgeon (Salensky) is developed from behind forwards. 



The chapter on the germinal layer is largely occupied with a 

 criticism of the observations of W. B. Scott ; and the author con- 

 cludes by giving his adhesion to the doctrine of His, that the study 

 of the mechanical causes which affect the embryo, and the causal 

 connection of the changes which take place in the egg are the true 

 objects of embryology, and he points out that this side of the study 

 is to descriptive embryology what physiology is to zoology. 



White Corpuscles of the Blood.* — M. Eenaut describes the 

 different forms presented by the white corpuscles in different animals. 

 In the Crayfish, besides the ordinary lymph-corpuscles, there are 

 many larger bodies with well-defined nuclei, the protoplasm of which 

 contains large highly refracting granules, resembling in many 

 respects the vitelline granules of the Frog and other Batrachia. These 

 corpuscles have a sharply limited, but thin exoplastic pellicle ; and if 

 a drop of such lymph be allowed to fall into a drop of a 1 per cent, 

 solution of osmic acid, the white corpuscles are instantly fixed, with 

 their pseudopodia or protoplasmic processes extended ; and these 

 processes can then be seen to perforate the thin membrane, now black- 

 ened with the acid. There are thus two kinds of white corpuscles in 

 the Decapod Crustacea — the lymphoid corpuscles and the amoeboid 

 corpuscles. 



Do similar differences exist in the blood of Vertebrata ? 



In reply to this, M. Eenaut states that in the blood of all the 

 Vertebrata, from the Cyclostome to the Saurians, the white corpuscles 

 are of two kinds; one, the ordinary white corpuscle, composed of 

 hyaline protoplasm, presenting many short projecting points, with a 

 nucleus undergoing gemmation, and sending forth branched pseudo- 

 podia when placed under favourable conditions; the other containing 

 numerous brilliant granules imbedded in the protoplasm and sur- 

 rounding the nucleus. These resemble the second form of corpuscle 

 described above as existing in the lymph of the Crayfish, but differ 

 from them in having no outer limiting layer of condensed protoplasm, 

 or exoplasm, as Haeckel has named it. The application of osmic acid 

 shows that they may be subdivided into two other forms, one closely 

 analogous to cells undergoing transformation into fat-cells, which 

 present numerous granules, and stain black with osmic acid, and 

 another set which contains granules that are not fatty, but which 

 stain red with eosin. The best mode of demonstrating the existence of 

 these three forms is to fix the blood in the rete mirabile of the capil- 

 lary layer of the choroid in the posterior segment of the eye of a frog, 

 by removing the anterior segment and exj)osing it to the vapour of 

 osmic acid. At the expiration of twelve hours the eye is removed 

 from the vapour, washed, the chorio-capillaris detached from the 

 retina, and spread on glass ; it is afterwards coloui-ed with, and 

 mounted in, hsematoxylate of eosin. The corpuscles may then be 

 studied, and the three forms of ordinary, granular, and fatty corpuscles 

 can be easily distinguished. 



* ' Science,' ii. (1881) p. 505, from ' Arch, de Physiol.' and ' Lancet." 



