On the Structure and Development of Leptdosteus. 

 By F. M. Balfoor, LL.D., F.R.S., and W. N. 

 Parker.^ 



The authors commence this paper by thanking Professor 

 Alexander Agassiz for the material, both embryological and 

 adult, on which these researches were made. 



The first section is devoted to the general development. 

 In this section an account is given of the structure of the 

 ripe ovum, of the segmentation, of the history of the ger- 

 minal layers^ of the first development of the principal organs, 

 and of the external features of the embryo during embryonic 

 and larval life. The more important points established in 

 this section are — 



(1.) The ovum when laid is invested by a double covering 

 formed of {a) a thick inner membrane, the outer zone of 

 which is radially striated, and {b) an outer layer made up of 

 highly refractive pyriform bodies which are probably meta- 

 morphosed follicular epithelial cells. 



(2.) The segmentation is complete, though very unequal ; 

 the lower pole being very slightly divided into segments, and 

 its constituent parts subsequently fusing together to form an 

 unsegmented mass of yolk, like the yolk-mass of Teleostei. 



(3.) The epiblast is divided into an epidermic and a 

 nervous stratum, as in Teleostei. 



(4.) The walls of the brain, of the spinal cord, and of the 

 optic vesicles are formed from a solid medullary keel, like 

 that found in Teleostei. 



(5.) The lens, the auditory vesicle, and the olfactory pit, 

 are wholly developed from the nervous layer of the 

 epidermis. 



(6.) The segmental or archinephric duct is developed, as 

 in Teleostei, from a hollow ridge of the somatic mesoblast, 

 which becomes constricted oflP, except in front; thus forming 

 a duct with an anterior pore leading into the body cavity. 



The section on the general development is followed by a 



1 Tills paper is an abstract of one wliich will be published in full in the 

 ' Philosophical Transactions of the Kojal Society.' 



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