40 SUMMARY OF CURRENT RESEARCHES RELATING TO 



water have no envelope. In the Lake of Plon the egg-laying usually 

 begins in June and continues through the summer. 



The first cleavage divides the ovum into a larger and a smaller cell r 

 and the four-cell stage shows the characteristic Lamellibranch state — 

 three equal cells and one much larger. The first two cleavage-planes 

 are said to have little import as regards the future orientation of the 

 larva ; at the 49-cell stage the bilateral symmetry becomes distinct. 

 To discuss the details of the cleavage without diagrams is unprofitable ; 

 but we may note that the author distinguishes, towards the end of seg- 

 mentation, nine different cell-complexes, which represeut definite regions 

 of the larval body. The occurrence of cavities in the embryo, which 

 enlarge, disappear, and reappear, is noted ; the interpretation of these 

 as due to the hindering of excretory exosmosis by the egg-envelopes 

 will not apply to the naked egg of Breissensia. 



The differentiation of the larva begins after the invagination of the- 

 endoderm -portion ; the blastopore, at first wide, narrows to a short slit, 

 and then to a round hole ; the aichenteron is deep, and the shell-sac 

 invagination is very like it ; the blastopore is shunted forwards by the 

 multiplication of the component cells of the ventral plate ; stomach 

 and intestine are differentiated from the posterior portion of the endo- 

 dermic invagination ; where the blastopore was closed up the stomo- 

 daeum arises ; the proctodaeuru arises posteriorly in a similar fashion. 



The development of the velar area is traced, and the origin of the 

 mesenchyme and muscular tissue from two primitive somatoblasts, which, 

 however, also contribute to the endoderm. It is noteworthy also that 

 here, as in other molluscs, there is an internal migration of ectoderm 

 cells to assist in internal differentiation. The author ends his study 

 with a comparison of the larva of Dreissensia with the glochidium of 

 Unionidae and with the larvae of other Lamellibranchs. 



Arthropoda. 

 a. Insecta. 



Balloon-making Fly.*— Messrs. J. M. Aldrich and L. A. Turley 

 describes an interesting habit. On June 16th, 1899, while passing 

 along a country road near Moscow, Idaho, they saw some bright white 

 objects moving to and fro in the air at an elevation of eight or ten feet. 

 These were males of Empis poplitea, each carrying between its hind 

 feet a peculiar elliptical balloon-like structure about 7 mm. long (nearly 

 twice the length of the fly), composed of a single layer of slightly 

 viscid bubbles. In nearly every case there was a small fly pressed into 

 the front end of the balloon, apparently as food for the Empis, as 

 the attached forms (all dead) were partly Ghironomus, and partly 

 Oscinids and other Acalyptrate Muscids. 



The balloon seems to be made while the insect is flying, probably 

 by some modification of the anal organs, as in Aphophora and other 

 leaf-hoppers, and it is possible that the captured victim serves as a 

 nucleus in the balloon-making. One case of a captured fly without. 

 a balloon was observed. 



The purpose of the structure is to attract the female. It was seen* 

 that the males gathered in the path of an approaching female, and that. 



* Amer. Nat., xxxiii. (1899) pp. 809-12 (3 fig?.). 



