ZOOLOGY AND BOTANY, MIOKOSCOPY, ETC. 55 



streak appears, beginning at its anterior end, which developes the first 

 segments and appendages before the hinder portion is clearly defined. 

 Before the appearance of the primitive streak the mesoderm is divided 

 off from the small-celled ectoderm and at the same time nuclei, in- 

 vested by masses of protoplasm, emerge from the yolk-pyramids and 

 apply themselves to the mesoderm ; these masses seem to be derived 

 from the nucleus of the ovum and to have hitherto remained at the 

 centre. The mesoderm, like the primitive streak, developes first in 

 front. The conversion of the yolk-pyramids into endoderm, i. e. 

 into the epithelium of the mid-gut, only takes place when the embryo 

 is fully formed ; it commences during that stage which Prof. Metsch- 

 nikoff did not observe, and at the same time as the commencement of 

 flexion of the embryo. 



y. Arachnida. 



Poison-Apparatus of Scorpions.* — M. Joyeux-Laffuie finds that 

 the poison-organ of the scorpion (S. occitanus) is formed by the last 

 abdominal segment, where two small oval orifices serve for the exit 

 of the poison : there are two glands, equal in size, and symmetrically 

 arranged ; each occupies a space, covered externally by the chitinous 

 skeleton, and having internally an anterior and a posterior mem- 

 brane, formed by striated muscular fibres, which are inserted into the 

 chitinous skeleton. By their contraction the poison is forced out- 

 wards. The wall of the gland consists of a delicate layer, formed by 

 cellular tissue and smooth muscular fibres : on its internal surface 

 there are projecting lamellae, which increase the extent of the 

 secreting surface ; below this, is a layer of prismatic ceils, which 

 are filled with protoplasm containing in suspension and in abundance 

 fine rounded granulations, which are characteristic of the poison of 

 the scorpion, and hide the nuclei, which only become apparent on the 

 addition of acetic acid ; these are the cells which elaborate the poison, 

 and from which it escapes, by the rupture of the cells, into the central 

 cavity of the organ. 



Physiologically, this poison is very active, and that in direct 

 relation to the quantity introduced ; one drop is soon fatal to a rabbit 

 and still more active on a bird ; seven to eight frogs may be killed by 

 one drop, and the hundredth part of one is fatal to an ant of large 

 size. It would appear to affect the nervous system, and has undoubt- 

 edly a marked action on striated muscle, suppressing spontaneous 

 and reflex movement. 



Snares of Orb-weaving Spiders. f — The Eev. H. C. McCook, ac- 

 cepting Thorell's arrangement of the true spiders into Sedentary 

 (remaining for the most part in or upon their web and capturing 

 their prey by means of snares), and the Wandering (hunting their food 

 on the ground, the water, or trees), applies this principle of arrange- 

 ment according to economy to the first section of the Sedentary 

 Spiders — the Orb-weavers — which, whether " simply tentative, and in 

 its present form incomplete, is given with the hope that it may lead to 



* Comptes Kendus, xcv. (1882) pp. 866-9. 



t Proc. Acad. Nat. Sci. I'hilad.,.1882, pp. 254-7 (1 fig.). 



