112 



Dr. F. M. Balfour and Mr. AY. N. Parker. [Dec. 8, 



the size of the mandibular suspensorium ; the basi-hyal is very large, 

 is composed of two parallel pieces, and is very ITyxinoid. 



Brief and imperfect as this "Abstract" is, I trust it is sufficient to 

 show the extremely interesting and suggestive nature of this type ; 

 anyhow, no clear understanding of the morphology of this type of skull 

 can be had unless it be seen in the light derived from that of the 

 Elasmobranchs, the Sturgeon, and the Anurous larva on one hand, and 

 that of Amia calf a and the Teleostei on the other. 



III. " On the Structure and Development of Lejridosteus" By 

 F. M. Balfour. LL.D., F.R.S., and AY. N. Parker. Re- 

 ceived November 24, 1881. 



(Abstract.) 



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 germinal 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 np of highly refractive pyriform 

 bodies which are probably metamorphosed 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 archmephric duct is developed, as in 

 Teleostei, from a hollow ridge of the somatic mesoblast, which becomes 

 constricted off, except in front ; thus forming a duct with an anterior 

 pore leading into the body cavity. 



