ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 19 



Arthropoda. 



Muscle-Attachment and Origin in Arthropods.* — E. Sinethlage 

 deals with this subject in a number of Insect orders, some Crustaceans, 

 and Arachnoids. He concludes that the muscles are not fixed to the 

 hypodermis, but directly to the chitin of the outer skeleton. Laterally, 

 the plasma envelope surrounding the muscle-cell passes without definite 

 boundary into the plasma of the hypodermis, the basal membrane of the 

 latter passing round and forming the sarcolemma. Cells lying imme- 

 diately beneath the chitin give rise both to muscle-fibrils and chitin. 

 They may be regarded as epithelio-muscle-cells, since they subserve the 

 functions of both tissues. It is stated that in Artemia salina the muscu- 

 lature of the extremities is differentiated from the hypodermis, and is 

 thus ectodermal. Examination of developmental stages did not reveal 

 anything which might be regarded as mesoderm in the sense of a special 

 germinal layer. 



a. Insecta. 



Poly-embryony in Parasitic Hymenoptera.t — P. Marchal has found 

 more instances of poly-embryony or germinogony. After fertilisation, 

 and before the least hint of the germinal layers, the egg is dissociated 

 into a number (12-100) of germs, which proceed to develop. This 

 " specific poly-embryony " occurs in the Chalcidiidse in Ageniaspis 

 fuscicollis (Dalm.) Thorns. = Metjrtus fuscicollis (Dalm.), and in A. 

 testaceipes. It also occurs in Proctotrypidse (Polygnotus minutus). 

 Full details are given of the remarkable development. Marchal thinks 

 that in experimental poly-embryony, in accidental or teratological 

 embryony (true twins, double monsters), and in specific poly-embryony, 

 the factors are the same, and that the chief one is a change in osmotic 

 pressure at the beginning of segmentation. It is noteworthy that the 

 individuals which arise from one ovum are of the same sex. 



Poly-embryony in a Hymenopterous Parasite.J — F. Silvestri 

 describes the remarkable peculiarities of development in Litomastix 

 truncatellus Dalm., one of the Encyrtidae, which lays its eggs in those of 

 Fhisia gamma. The ova may be parthenogenetic, or they may be 

 fertilised. In both cases there are two polar bodies formed, and the 

 first divides into two. The three unite into one, which seems to share 

 in the development. The parthenogenetic ova give rise to males only ; 

 the fertilised ova give rise to females only. Two kinds of larvse — 

 asexual and sexual — are distinguished, and each egg may give rise in 

 two different ways to a number of larvae of both kinds. We have 

 found it difficult to follow the author's account, but a less condensed 

 memoir is promised. Three points are emphasised — poly-embryony, the 

 .peculiar history of the polar bodies, and the dimorphism of the larvae. 



Morphology of the Insect Head.§— S. Bengtsson maintains, with 

 -especial reference to the larva of Phalacrocera, that the insect head has 



* Zool. Jahrb. Abt. Anat., xxi. (1905) pp. 495-514 (2 pis.), 

 t Arch. Zool. Exper., ii. (1904) pp. 257-335 (5 pis.). 

 % Atti R. Accad. Lincei Roma, xiv. (1905) pp. 534-42. 

 § Zool. Anzeig., xxix. (1905) pp. 457-76 (5 figs.). 



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