ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 599 



length of ovum 1-5, in P. capensis 0*5-0 # 6 mm., of P. balfouri 

 • 4-0 • 5 mm., and of P. edwardsii * 04 mm., a perfect series in regard 

 to size and amount of yolk. 



As to the segmentation it is not only to be noted that it is not a 

 true segmentation, but also that no part of the nucleus or centre of 

 force of the unsegmented ovum enters the clear endoderm masses ; 

 when endodermal nuclei do appear, they are larger than the ecto- 

 dermal and are very irregular in shape ; dividing directly, they do not 

 exhibit the usual karyokinetic* figures. In other words, there are 

 two different modes of segmentation, neither of which are instances 

 of complete cleavage in the ordinary acceptation of the term. The 

 first kind is preceded by the division of the nucleus of the fertilized 

 ovum and its products, and this gives rise to the ectodex*m cells ; the 

 second, which takes place contemporaneously with the first, divide 

 the larger and cleaner vegetative part of the ovum with the endoderm 

 masses. Inasmuch as the gut is to be looked upon as a vacuole, it 

 resembles in all essential respects the cavity in the body of a ciliated 

 infusorian. 



After a full account of the nucleus of the unsegmented ovum, the 

 male and female pronuclei and the endodermal nuclei are described 

 in detail ; the structure of the gastrula and the formation of the 

 mesoderm are discussed, and, in conclusion, the author extends the 

 results which flow from his discovery of the syncytial nature of 

 Peripatus ; if they are of general truth we must modify our ideas 

 about the ancestral metazoon, and, instead of looking on it as a 

 colonial protozoon, regard it as having the nature of a multinucleated 

 infusorian, with a mouth leading into a central vacuolated mass of 

 protoplasm. In centrolecithal eggs it has already been observed that 

 in early stages separation was incomplete, but the ordinary explanation 

 that this phase is only temporary is not confirmed by Heathcote's 

 discovery that there is no sej>aration in the myriopod lulus. 



5. Arachnida. 



Development of Agelena usevia.* — After a short review of the 

 comparatively few works on the subject, Mr. W. A. Locy gives an 

 account of his own researches. 



1. The eggs. — Eggs in the fresh state were studied when im- 

 mersed in pure oil ; external features were also observed on eggs 

 hardened in alcohol, after removing the shell and clarifying with oil 

 of cloves. The most satisfactory method of preparing them for 

 sections is to heat the eggs in water to 80° C, and after being slowly 

 cooled, to pass them through a graduated series of alcohols. Perenyi's 

 fluid produces an alteration in the yolk, but is useful in conjunction 

 with other methods. Corrosive sublimate renders the eggs too 

 brittle. 



Grenacher's borax-carmine is the best staining agent, though in 

 the later stages the egg has to remain in the fluid for a considerable 



* Bull. Mus. Comp. Zuol. Cambridge, xii. (188(3) pp. 63-95 (12 pis.). 



