ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 565 



reticulum are filled by a fine protoplasm, which is perhaps comparable 

 to the gi'anular substance of the neuroglia of Vertebrates. 



The close relation between the hypodermic epithelial cells and 

 their prolongations with their nerve cells and fibres, together with 

 the absence of any histological differences between the two sets of 

 fibrils, are especially interesting as calling to mind the characters of 

 the nervous system of larval Annelids. 



Varieties of Branchiobdella varians.* — W. Voigt has a careful 

 study on the variations of B. varians. 



He finds that we have here to do with an animal which may 

 be of great importance in our knowledge of the mode of origin 

 of species. He shows that it is on the very point of giving rise 

 by its varieties to new species. The so-called B. parasita is 

 undoubtedly the form from which the others have been derived. The 

 fact that the variety liexodonta found on the gills of crayfishes in 

 North Germany is replaced in South Germany by the variety astaci 

 points to external influences as being the cause of this local dis- 

 tribution ; the differences may be supposed to be due to temperature 

 or to the qualities of the water, or the bodies dissolved therein. To 

 such suppositions there are, however, powerful objections, and we 

 must therefore look for the causes of variation in the animals them- 

 selves. Differences have been observed in the size of the ova, and in 

 the characters of the dissepiments between the segments which carry 

 the segmental products ; with these other differences appear to be 

 correlated, but their exact relations have as yet to be carefully 

 worked out. 



Ovum and its Fertilization (in Ascaris).t — The discrepancies in 

 the recorded observations of fertilization in the ova of Echinoderms 

 led E. van Beneden to study the subject in fresh types, and finally to 

 pursue in the Ascaris megalocephala of the horse the important series 

 of observations which he has recently published in great detail. 

 The memoir is divided into four descriptive chapters and a general 

 summary. 



The first chapter describes the constitution of the ovum and 

 spermatozoon. 



The advantage to be obtained from studying the ovum in this 

 Nematode is that in the uterus and oviduct definite stages of fertiliza- 

 tion constantly occur at definite points. 



On quitting the ovarian rachis the previously bilateral ovum 

 acquires an elliptic form, and — at the point of previous attachment 

 — shows a micropyle, underlying which is a naked protoplasmic 

 process, the 2>/«{/ of impregnation, situated on a polar disk forming a 

 slight eminence on the transverse (or short) axis of the ellipsoid. 

 Ultimately a delicate membrane comes to cover the ovum except at 

 the micropyle. Within the so-called " nucleus," or yerminal vesicle 

 (which is bounded by a membrane), is the " nucleolus," or germinal 

 corpuscle, consisting of two disks and situated peripherally on the 



* Arbfcit. Zool. IjiBt. Wurzburg, vii. (1884) pp. 41-94 (2 pis,), 

 t Arch, (le liiol., iv. (1883) pp. 2C5-(jlO (1 pi.). 



