488 



SCIENCE. 



[N. S. Vol. XVII. No. 430. 



notata possess chromosomes which are suffi- 

 ciently different, morphologically, to be 

 distinguished from each other in the cells 

 of the hybrid between the two species. The 

 former has long, straight chromosomes; 

 the latter short, slightly curved ones. 



These two kinds of chromosomes retain 

 their individuality during the development 

 of the hybrids to a late cleavage stage, as 

 far as any attempt was made to follow 

 them. During the first two cleavages each 

 kind remains grouped upon the spindle. 

 During the third cleavage this grouping 

 has largely disappeared and the two kinds 

 of chromosomes occur mingled upon the 

 spindle. During the later cleavage stages 

 this bilateral distribution of the chromo- 

 somes has altogether disappeared. The 

 two kinds, however, can readily be distin- 

 guished, but thoroughly mingled. 



Homologies of Anterior Limb : Theo. Gill, 



Smithsonian Institution. 



The homologization of the anterior mem- 

 iber of the terrestrial vertebrates with that 

 of fishes is a problem involving a greater 

 diversity of interpretation than any other 

 structure. By the early anatomists 

 (Cuvier, Owen, Stannius) bones which are 

 now universally regarded as parts of the 

 shoulder girdle were designated as the 

 humerus, radius and ulna. 



It is contended that Polypterus gives us 

 a key to the problem in question, as was 

 urged by the speaker in 1872, 1878 and 

 1882. 



The diverging branches which inclose the 

 flat cartilage with which the aetinosts or 

 basal bones of the fin connect are homo-- 

 logues of the radius and ulna; the tuber- 

 cular process of the coracoid cartilage with 

 which they articulate is the representative 

 of the humerus ; the cartilage between the 

 diverging processes is the stuff from which 

 the carpal bones are developed; and the 

 aetinosts represent the metacarpals. The 



nearly similar conclusions of Emory (1887) 

 and Pollard (1892) were much later and 

 somewhat different. 



Pollard found the humerus, radius and 

 ulna in the same parts as the speaker. He 

 went to an extreme, however, in the homol- 

 ogization of the intermediate cartilage or 

 'mesopterygium.' This^ he thought, 'forms 

 probably the 'intermedium and centralia, 

 and the chief foramen in the ossified part 

 represents the intercarpal foramjen.' 



Inasmuch as Polypterus is a very special- 

 ized modern form of the great crossopteryg- 

 ian series, and no extinct representatives of 

 its phylum since Devonian times have been 

 discovered, such an extension of homologies 

 is not legitimate and we must be content to 

 recognize the 'mesopterygium,' as a swhole, 

 to be homologous with the carpus. This 

 is in accord with the most recent, investi- 

 gations, but still m\ist be confirmed by 

 paleontology. 



Homologies of the Centronucleus: Gary 

 N. Calkins, Columbia University. 



The Structure of the Ostracoderms: W. 

 Patten, Dartmouth College. 



1. In a newly acquired specimen of Tre- 

 mataspis the post-orbital and the two pairs 

 of marginal openings are completely closed 

 by a small number of close-fitting poly- 

 gonal plates. In Cephalaspis a single pair 

 of very large marginal openings, closed in 

 a similar manner, has been found. A large 

 marginal opening has also been found in 

 Thyestes. 



2. In the same specimen of Tremataspis 

 the dumb-bell-shaped orbital opening is 

 closed by a polished layer of shell, con- 

 tinuous with that of the dorsal shield. 

 Over the lateral ends of the opening the 

 shell is partly broken, but shows clearly 

 that it formed a complete dome-shaped 

 cover to each eye. 



3. In Bothriolepis, the large median or- 



