590 SUMMARY OF CURRENT RESEARCHES RELATING TO 



nerve-ring round the blastopore ; it is quite possible that tliis branch 

 is much younger than that of insects. 



7. Embryology does not yet enable us to say definitely whether the 

 Hexapoda and Myriopoda form a natural group, but the evidence of 

 anatomy is in favour of their being so ; the difficulties presented by 

 the Chilognatha are not yet solved. The Hexapoda must have 

 branched off early from the common stem. At present the two sets 

 may be kept together under the name of Insecta ; as for the rest, they 

 are best compared a posteriori, and not a priori. 



Embryology of Insects and Arachnids.* — The embryology of 

 Arachnids shows many resemblances to that of insects. Mr. A. T. 

 Bruce has traced the development of Thyridopteryx ephemerseformis 

 from early segmentation to an advanced embryonic stage. In the 

 earliest stages cells are found in the yolk, whence they emigrate to 

 form a blastoderm ; the egg therefore is not truly centrolecithal. 

 Some of these cells never reach the surface, but remain as " yolk 

 bulbs." 



In the grasshopper all the cells emigrate, and the yolk is arranged 

 in pyramidal masses. 



The embryo in insects is formed from a thickening of the surface 

 of the egg, like the primitive annulus of spiders. 



The endoderm and mesoderm arise partly by invagination and 

 partly by delamination along the middle line ; the yolk-cells appear 

 to have nothing to do with the formation of endoderm. No "dorsal 

 organ " like that described by Brandt in the Neuroptera, was 

 observed in the insects studied — including Lepidoptera, Coleoptera, 

 and Orthoptera. 



The nervous system arises as two ectodermic strings at the sides 

 of the blastopore. 



The supra-oesophageal ganglion in Thyridopteryx consists of two 

 parts, the posterior of which supplies the paired labrum, the anterior, 

 the antennae. 



In the grasshopper both maxillae have two lobes at the base of 

 the main axis, recalling the exopodite and epipodite of crustacean 

 appendages, though they are probably not homologous. 



In one stage of the spider embryo an abdominal appendage is 

 being converted by invagination into a lung-book. 



The amnion in Thyridopteryx forms part of the dorsal surface of 

 the body. 



Tracheal invaginations occur in the maxillary segments of the 

 grasshopper. 



a. Insecta. 



Spermatogenesis.! — Continuing his classical researches on sper- 

 matogenesis, Prof. v. la Valette St. George describes the development 

 of the sperms in Blatta germanica. The male glands, which have 

 the form of four transparent vesicles lying in the last segment, 

 and provided with a fine efferent duct, are surrounded by white fatty 



* Johns-Hopkins Univ. Circ, v. (1886) p. 85. 



t Arch. f. Mikr. Armt., xxvii. (188(3) pp. 1-12 (2 pis.). 



