OF ARTS AND SCIENCES. 43 



former being the female and the latter the male. When 

 young, both sexes look exactly alike. He had established a 

 new genus, Heterandria, in which the sexual differences were 

 very remarkable ; the position and shape of the ventral and 

 other fins being quite different, which he showed by diagrams. 

 ,The habits of these fishes, living in immense numbers, crowd- 

 ed together in very shoal water, enabled him to explain a 

 figure represented in the fifth volume, Plate 41, of his Fossil 

 Fishes, in which the great number of individuals was remark- 

 able ; and the knowledge of the sexual differences renders un- 

 necessary any hypothesis to account for supposed displace- 

 ments of fins, or the occurrence together of different species. 

 He also had established a new genus, Zugonectes, in which 

 no sexual differences existed. 

 Dr. Burnett read a paper 



" On the Signification of Cell- segmentation, and the Relations of 

 this Process to the Phenomena of Reproduction. 



" The phenomena of the segmentation of cells are intimately con- 

 nected with many of the highest conditions of organization, and it be- 

 comes a question of no little interest in physiology, what interpre- 

 tation is to be put upon this process of segmentation. 



" By the term Cells, I include, not merely the elementary constituent 

 particles of organized forms, but also ova, for it now appears pretty 

 definitely settled that the ovum xs, morphologically, only a cell; of this 

 point, deducible from the observation of various naturalists upon the 

 elementary condition of ova in different lower animals, I have re- 

 cently satisfied myself, from investigations upon the ovaries of insects. 

 Moreover, the segmentation of the ovum as preliminary to the for- 

 mation of a new individual involves physical phenomena not in the 

 least different from those of this process occurring with simple indi- 

 vidual cells. 



" This process consists, as is well known, in the successive halvings 

 of the nucleus of a cell, the number of the parts produced being, 

 therefore, whether greater or less, the multiple of two in a geometri- ' 

 cal progression. Its physical conditions are, briefly, first, a sulcation 

 of the cell membrane at one point ; the concavity thus commenced 

 gradually deepens and extends through the cell, ending in the com- 



