ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 449 



shores of the Falkland Islands. As in the related form, Modiolarca 

 trapezina, there is a brood-chamber. The eggs pass into the inner gills, 

 are fertilised, and develop there till they have attained their definitive 

 form. The various systems — nervous, muscular, alimentary, and so on 

 — are described at length. 



Relation between Body and Shell in Bivalves.* — R. Schwarz 

 has studied this problem in a large number of cases. He finds two 

 styles of architecture, the one represented by Gardium, Venus, Cytherea, 

 Tapes, Artemis, etc., and the other by Mytilus, Avicula, Pecten, Ostrea, 

 etc. He also deals with aberrant forms, such as Lima (which belongs 

 to the Gardium style, not to that of Pectinids), Hippopus, and Tridacna, 

 and seeks in general to show that the study of the shell apart from the 

 body is sure to mislead. More definitely, as he says, the assumption 

 that the shell-margin or the hinge in one bivalve is homologous with the 

 shell margin or the hinge in another, must be entirely given up. 



Concentration of Nervous System in Lamellibranchs. f t — Paul 

 Pelseneer describes the close approximation of the ganglia in Lima 

 squamosa, where cerebral, pedal, and visceral ganglia are close together 

 below the oesophagus. In L. inflata the arrangement is intermediate 

 between that of L. squamosa and the typical wide separation. 



Arthropoda. 

 «• Insecta. 



Accessory Chromosome in Aplopus Mayeri.f — H. E. Jordan has 

 traced the history of the accessory chromosome in this Phasmid. Some 

 spermatozoa have 18 chromosomes, like the ova ; others have one less. 

 The accessory chromosome appears in the resting stage of the secondary 

 spermatogonia as a chromatin nucleolus characteristically close to the 

 nuclear wall. At the last spermatogonial division it passes over into the 

 resting stage of the primary spermatocyte without entering a reticular 

 stage as do the ordinary chromosomes. Both the primary and secondary 

 spermatogonia have a metaphase group of 35 chromosomes. Metaphase 

 groups of follicle cells of the ovary contain 36 chromosomes. 



Synapsis occurs in the early stages of the growth period by an end- 

 to-end union of pairs of univalent elements. Equatorial plates of 

 primary spermatocytes contain 18 chromosomes. The accessory chromo- 

 some passes undivided to one pole of the first maturation spindle, and 

 thus produces a dimorphism of the daughter-cells and the resulting 

 spermatozoa. 



The first maturation division is reductional ; the second isequational. 

 Equatorial plates of secondary spermatocytes show a disparity in the 

 number of chromosomes. One group contains a large U-shaped element 

 peripherally, and numbers 18 ; those groups lacking a body of such form 

 contain only 17 chromosomes. The accessory chromosome can be traced 



• Morphol. Jahrb., xxxviii. (1908) pp. 91-134 (3 pis.). 



t Bull. Classe des Sciences Acad. Roy. Belgique, Nos. 9-10 (1907) pp. 871-8 

 (3 figs.). { Anat. Anzeig., xxxii. (1908) pp. 281-95 (48 figs.). 



Aug. 19th, 1908 2 h 



