272 SUMMARY OF CUEEENT EESEAECHES EELATING TO 



origin of the two stocks must be put far back is shown by the absence 

 in the annelid trochophore of any specifically molluscan trait, no less 

 than by the entire absence of segmentation in the molluscan trocho- 

 phore. 



Development ?of Cyclas.* — Dr. J. Meisenheimer has studied the de- 

 velopment of the heart, pericardium, nephridia, and genital cells in 

 Cyclas, as a continuation of his work on Dreissensia. He finds that the 

 first trace of these organs is a cluster of cells which arises in connection 

 with the external body-wall. This rudiment comparatively late in de- 

 velopment becomes separated from the body-wall, and a few cells are 

 differentiated as genital cells. Next the nephridial vesicle is separated 

 off at either side, and as it develops further, the remaining portion of 

 the rudiment shows first the lower and then the upper pericardial space. 

 The genital cells increase in number, the nephridium opens into the 

 pericardial space and also to the exterior, the right and left halves of 

 the pericardial space fuse above and below the gut, and the parts 

 gradually acquire the adult condition. The special point is that there 

 is no " mesoblastic streak " in Ziegler's sense, but merely a common 

 rudiment of genital cells, nephridium, pericardium, and heart, which 

 are differentiated in this order. A similar condition of affairs is now- 

 known to occur in Dreissensia, Cyclas, and Paludina, which justifies the 

 conclusion that in Mollusca generally, apart from Cephalopoda, the four 

 structures, heart, pericardium, nephridia, genital cells, originate from 

 a common rudiment which arises from the external cellular wall, and 

 behaves as an independent unit throughout its development. In the 

 four genera named the order of development varies, as does also the 

 place of origin of the individual organs, but these are to be regarded 

 only as special modifications of a primitive type. 



Malformed Specimens of Anodonta cygnea.f — Mr. H. H. Bloomer 

 concludes from an examination of three strangely malformed specimens, 

 that the animal is able to repair even extensive damage to the mantle 

 lobes, but is not able to make good injuries to the gills. The mussel 

 seems able to live and thrive with very much aborted respiratory organs, 

 and with considerable displacement of the various internal organs. 



Arthropoda. 

 a. Insecta. 



Oogenesis in'Q,ueen Bee. J — Wilhelm Paulcke has studied the ovarian 

 tubes in Apis meUifica ? > with special reference to the relations of epi- 

 thelial cells, yolk-cells, and ova. The proximal ends of the tubes contain 

 undifferentiated nuclei imbedded in a common protojdasm. Following this 

 region is one called the synapsis-zone by the author, from the condition 

 of the nuclear chromatin ; in this region the undifferentiated nuclei which 

 later give rise to the cells of the follicular epithelium, are distinguishable 

 from the primitive germ-nuclei which give rise to yolk-cells and ova. The 

 zones are not separated by any lino of demarcation, so that the synapsis- 

 zone shows the beginning of a process clearly marked in the next or 



• Zeitschr. wiss. Zool., lxvi. (1901) pp. 417-28 (1 pi. and 9 figs.), 

 t Joum. Malacol., vii. (1900) pp. 186-8 (1 pi. and 2 figs.). 

 t Zool. Jahrb., xiv. (1900) pp. 177-202 (4 pis. and 1 fig.). 



