DEVELOPMENT OF KISIIES. 299 



joined by tlie precaval vein (e), becomes first a pulsating tube, and 

 afterwards, by extension, constrictions, and intervening dilatations, 

 an auricle, a ventricle {k), and a bulbus arteriosus ; from which the 

 hyoid vascular arch (u), succeeded by the branchial vascular arches, 

 pass upwards to communicate with, and impart new vigour to the 

 flow of blood in, the primitive longitudinal dorsal vessel. The 

 ' laminti3 ventrales,' continued down the sides of the head, form 

 vertical folds or arches, the interspaces of which are converted into 

 clefts communicating with the commencement of the alimentary 

 canal. The first of these arches is the largest, and from its blas- 

 tema the mandibular and hyoidean arches, with their appendages, 

 are developed ; the five succeeding arches are the true branchial 

 ones. The metamorphoses of the corresponding vascular arches, and 

 the development of the gills, have been already explained, (p. 265.). 

 The jugular (b) and cardinal (m) veins unite to form the precaval (e), 

 which joins the hepatic and vitelline veins in a common trunk, or 

 sinus, opening into the auricle. 



The first three successive enlargements at the fore jjart of the 

 neural axis are connected respectively with the olfactorj^, optic, and 

 acoustic nerves ; the first (p) becomes, then, divided into prosen- 

 cephalon and rhinencephalon ; the second (o) rapidly gains superior 

 bulk in connection with the large and early-developed eyes ; and the 

 pineal and pituitai-y appendages appear. The cerebellum is the last 

 part which is formed, upon the epencephalon (a). 



In tlie mean while the liver (/) has been developed from the back 

 part of the intestinal neck of the vitellicle ; the pancreas being a later 

 pullulation of cfeca fi'om the intestine itself. The kidneys ( p) and 

 generative glands (o) are formed out of blastema beneath the pi-imitive 

 vascular sinus ; but their ducts are caecal developments from the pos- 

 terior part of the intestine : this latter stage of development is never 

 attained, as regards tlie generative organs, in the Dermopteri. Ratlike* 

 detected in the embryo Blenny, what Carus afterwards found in the 

 human foetus, germinal vesicles in the nascent ovarium prior to birth, 

 — two generations successively included in the parent. 



The ureter (^) always makes its appearance very early before tlie 

 embryo quits the ovum; it communicates with the exti'eniities of the 

 transverse parallel tubuli uriniferi formed by confluence of primitive 

 cells in the renal blastema. The cardinal veins traverse or gi-oove 

 the kidneys, as they do the Wolffian bodies in the embryos of higher 

 vertebrata ; and this primitive relation of the vascular to the renal 

 system is not changed in fishes by the substitution of true kidneys for 

 the primordial renal organs.t 



* CXXVIII. 



f Von Baer appears to have first appreciated this interesting homology. " Alles 



