276 



COMPARATIVE ANATOMY 



formed as a series of paired solid thickenings of the somatic 

 mesoblast to the inner sides of the segmental ducts. The 

 solid thickenings are converted into coiled tubules which 

 grow towards and acquire openings into the segmental ducts. 

 Their distal ends dilate to form hollow vesicles, each of 

 which is invaginated by the ingrowth of a small plexus of blood- 

 vessels, and gives rise to the structure known as a Malpighian 



jnes 



Fig. 69 



A. Diagram of the excretory system of a tadpole about 12 mm. 

 long, showing the pronephros and origin of the mesonephric 

 (Wolffian) tubules. K. Diagram showing the kidneys and repro- 

 ductive organs of a frog at the end of the metamorphosis. 

 The broken line represents approximately the position of the 

 strip of peritoneal epithelium which gives rise to the oviduct, cl. 

 cloaca; <1.ao, dorsal aorta; _/"/', fat-body; gi, glomerulus ; g~r, 

 genital ridge ; mes, mesonephros ; ms.t, mesonephric tubules ; 

 od, oviduct ; m^f, position of oviducal opening ; pnf, pronephric 

 funnels ; //, pronephros ; sg, segmental duct. 



body. The tubule grows out beyond the Malpighian body 

 towards the peritoneal epithelium, fuses with it and opens into 

 the ccelom by a ciliated funnel or nephrostome. According to 

 Marshall, the nephrostomes afterwards lose their connection 

 with the mesonephric tubules and open into the renal veins. 

 The whole organ is compacted by the ingrowth of abundant 

 vascular tissue, the mesonephric tubules increase in number 

 and branch freely, and a solid glandular mass, the kidney 

 or mesonephros of the adult frog, is formed on either side. 

 Thus before and nearly up to the time of metamorphosis the 



