456 Mr. E. E. Prince on the 



the thick basilar plate. This plate unites with the trabeciilse 

 in front, and the tloor of the cranium is thus completed, while 

 the walls and roof are still membranous. The notochord, on 

 penetrating the skull, bends down very suddenly at an angle 

 of about 90°, and the basilar plate bends down likewise, but 

 passes forward at a more moderate angle — this declination of 

 the spheno-occipital plate or basilar cartilage producing a 

 flexure of the cranial region, which is much greater tban 

 usually supposed ; indeed, the floor of the skull, soon after the 

 embryo emerges from the ovum, lies in a plane almost 

 parallel to the plane of the branchial arches. 



Stomoclceum and Proctodceum. 



The preceding skeletal structures are usually well developed 

 within a week or ten days after hatching ; but it is not until 

 that time is completed (the seventh day or later) that the 

 stomodseum is externally open. The anus is still later. Nor 

 is this surprising, as the embryo derives all the needful nutri- 

 ment from the store of yolk which protrudes so prominently 

 on the ventral aspect of the body during these early stages. 

 Its bulk, however^ continuously diminishes, and soon after the 

 anal aperture arises it wholly disappears. The anus in all 

 the forms under consideration appears comparatively late. 



The anal tract is thus a solid cord until the lumen of the 

 mid-gut extends into it, the communication of the proctodseum 

 and the anterior portion of the alimentary canal being appa- 

 rently incomplete until the tenth or fourteenth day after 

 liberation. 



Hcevial Si/ stem. 



The vascular system cannot be treated in detail in this 

 place ; but one point demands some reference. As already 

 stated, the heart's pulsations ccmmence at an early embry- 

 onic stage, long before a true hsemal circulation exists. It 

 can hardly be doubted that a colourless plasma is distributed 

 over the trunk of the embryo, though it is impossible to detect 

 any such lymph-circulation. At a certain late stage red cor- 

 puscles do make their appearance, though whence they are 

 derived is a question as yet undecided. Many considerations 

 favour Ryder's view that they are directly periblastic, and 

 some evidence, from observations on Alosa *, Salmo, and 

 GastrosteuSj seems to support it. Sections of early embryos 



* In the embryos of this species Ryder affirms that the venous end of 

 the heart opens into the persistent segmentation cavity (U. S. Fish. Comm. 

 Eep. 1882, p. 537). 



