74 Miscellaneous. 



Observations on the Embryology of Insects and Arachnids. 

 By A. T. Beuce. 



The work, of which a short abstract is^ here given, comj^rises ob- 

 servations extending over a period of nearlj- two years. 



A more detailed and illustrated account now in course of pi-epa- 

 ration will, it is hoped, show that if these observations have not 

 brought to light anything absolutely new they have at least thrown 

 additional light on several important questions in insect embryo- 

 logy. 



With insect-eggs, the opacitj' of which renders them unsuitable 

 for superficial observation, the sectional method leads to the best 

 results. This method was followed in these investigatio-ns. 



The important points to be determined in insect embryology are 

 the segmentation of the egg and the formation of the blastoderm, 

 the origin of the embryo and embryonic membranes, the formation 

 of the germinal layers, metameric segmentation and all connected 

 with it, including number of appendages, nerves, ganglia, &c. 



The embryology of xirachnids, or at least of spiders, sliows many 

 points of resemblance to the embryology of insects. The first trace 

 of the spider-embryo, the so-called primitive cumulus, is not unlike 

 the early embryo of the Orthoptera. In the head region of the 

 advanced spider-embryo are folds which very closely resemble the 

 amniotic folds of the insect-embryo. 



The insects studied included representatives from the Lepidoptera, 

 Coleoptera, and Orthoptera, while a few incomplete observations 

 were made on the embryology of the Neuroptera and on the matura- 

 tion of the ovum in Musca. 



The eggs of the spiders studied probably belonged to several 

 species. 



The embryology of Thyrklopteryx ephemerceformis, or the common 

 bag-worm, was carefully studied. Owing to abundance of material 

 its development was followed from the early stages of segmentation 

 to the advanced embryonic stage. 



The segmentation of the egg of Thyridopteryx corresponds to 

 that of the Lepidopterous insect described by Bobretzky. It can 

 hardly be called a centrolecithal segmentation, inasmuch as in the 

 earliest stages colls are found, not at the surface surrounding a central 

 yolk-mass, but lying in the yolk, whence they migrate to the surface 

 to form the blastoderm. 



In Thyridopteryx it appears that some of the primitive em- 

 bryonic cells never reach the surface, but remain as yolk-cells, round 

 each of which, in the later stages of embryonic development, an 

 aggregate of yolk-spherules occurs, and thus are formed the yolk- 

 balls or segments. 



In the grasshopper, however, there is a stage in which all the 

 undifferentiated cells are apparently at the surface, while the yolk 

 is arranged in pyramids corresponding to the yolk-jiyramids of 

 Artocus. 



