ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 69 



cells, whicli become attached to tlie ectoderraic portion of the alimentary 

 canal to form the median or endodermic region. 



(c) Derivatives of the mesodei-m. — In the marine Polyzoa the general 

 cavity is not lined by a mesodermic epithelium. The walls of the zooecium 

 only exhibit a single sub-skeletal layer of ectodermic cells. Scattered 

 mesoderm cells occur near the wall, attached on the one hand to the ecto- 

 derm, and internally to other cells forming a funiculus. These mesoderm 

 cells are better described as funicular than as parenchymal tissue. The 

 muscular system is described in detail. Those of the digestive tube or 

 polyj)ide are to be distinguished from the parietal and opercular muscles 

 of the zooecium. Histologically there are three groups: (a) muscles 

 composed of isolated filiform fibres, with undifferentiated contractile sub- 

 stance, covered by a delicate sarcolemma, best seen near the spherical 

 nucleus, which lies near the middle of the fibre; (b) muscles in which 

 the extremities enlarge at the punctum fixum into a long triangular band ; 

 (c) muscles of the principal retractor, transversely striated, but only 

 peripherally, so that the central substance remains undifferentiated. The 

 reprodactive organs are also mesodermic. The products fall into the cavity 

 of the zooecium, and thence into the tentacular sheath, which ruptures in 

 admitting them. 



(d) Development. — Herr Ostroumoff finally distinguishes and describes 

 three types of larvae : — (a) the ordinary type of Chilostomata, (6) the 

 Cyphonaut type, (c) the vesicularid larva or Ctenostoma type. A brief 

 notice of the metamorphosis (generally corroboratory of Barrois) concludes 

 all that has yet been published of this memoir. 



Arthropoda. 



Spermatogenesis of Arthropods.* — Herr H. de Wielowieyski reports 

 some of the results of his researches on Arthropod spermatogenesis, which 

 were independent of the investigations of Prof. Gilson, though mainly 

 corroborating his conclusions. 



1. Contrary to the opinion of several observers of vertebrate spermato- 

 genesis, Wielowieyski maintains that the chromatic nuclear filament of the 

 spermatocyte is not directly formed into the head of the spermatozoon, but 

 breaks up, becoming thoroughly distributed among the achromatic plasma. 



2. While Gilson explains the multinuclear spermatogonia as the result 

 of endogenous division, Wielowieyski maintains that they arise by the very 

 complete fusion of individual cells. 



3. In Asellus aquaticus Gilson notes how the mother sperm-cell divides 

 into two, of wrhich the one portion continues to divide into spermatocyte 

 nuclei embedded in the common protoplasm, vv^hile the outer half (which 

 he calls female) remains passive and undivided, finally, however, disinte- 

 grating. This suggestive history is denied in toto by Wielowieyski, who 

 maintains that here also the mother sperm-cell divides into separate 

 spermatocytes. The appearance Gilson described, he regards as due to 

 artificial confluence. 



4. In insects, Gilson described the developing spermatozoa plunged in 

 the remnant of the protoplasm of the spermatogonium, with one or more 

 (female) nuclei in the wall. The envelope round such a bundle appeared 

 to Wielowieyski, on the other hand, distinctly cellular, in fact an epi- 

 thelium. 



5. In conclusion he notes the presence of accessory cells occurring 



* Arch. Slav, de Biol., ii. (1886) pp. 28-36. 



