74 



CHORDATE ANATOMY 



Fig. 67. — Transverse sec- 

 tion through the urinogeni- 

 tal region of a four-day 

 chick embryo, g, mesoder- 

 mal epithelium (peritoneum) 

 of genital ridge; w, infolding 



The oviducts in elasmobranchs and probably some amphibians arise 

 by longitudinal splitting of the pronephric duct, one portion of it serving 

 thereafter as the mesonephric duct while the other portion acquires, 

 by fusion of several pronephric nephrostomes, a wide anterior opening 

 into the coelom in the vicinity of the ovary. In other vertebrates, the 



oviduct develops as a fold of peritoneum (Fig. 

 67, m) closely parallel to the Wolffian duct but 

 independent of it. 



The Mesenchyme. Reference has been 

 made (page 69) to the fact that certain regions 

 of the mesodermal somite, the sclerotome and 

 the dermatome, are the source of cellular 

 material which becomes detached from the 

 somite and aggregates in the spaces between 

 the somite and neighboring organs or layers 

 where it produces skeletal, connective and 

 integumentary tissues. This secondary meso- 

 derm ("derm" implying a sheet or layer), being 

 usually not disposed in definite layers, is called 

 mesenchyme. But the somite is not the only 

 of peritoneum to form Miil- source of mesenchyme. Quantities of it are 



lerian duct; ms, mesentery; Q^UCed in all rCgionS of the CmbryO. 



s, mesenchyme cells which i^ ° ■' 



give rise to the stroma 

 (non-genital tissue) of gonad; 

 t, mesonephric tubules; 



W, Wolffian duct. (From visceral layers of the hypomere are a prolific 

 Kingsiey, after Waideyer.) ^^^^^^ ^^ -^^ numcrous cells becoming detached 



from the outer (next the ectoderm) surface of the parietal layer and from 

 the inner (next the endoderm) surface of the visceral layer. Also the 

 endoderm contributes to the mesenchyme which accumulates between the 

 enteric wall and the adjacent layers of mesoderm. The ectoderm plays 

 a minor part but evidence has been found indicating that mesenchyme 

 of ectodermal origin, "mesectoderm," participates in the development of 

 parts of the skeleton of the pharyngeal region. 



Mesenchyme spreads from its place of origin and eventually is found 

 in all parts of the embryo. Although late in origin, its importance is 

 by no means secondary. Chief among its derivatives are the following 

 materials and structures. 



Fibrous connective tissue is omnipresent in the adult vertebrate. It 

 invests, supports, connects, separates or cushions parts of the body. 



Every location where cartilage or bone is destined to develop is occu- 

 pied by mesenchyme. The deeper parts of the skull, the vertebral 

 column, ribs, sternum and the skeleton of the paired appendages are 

 first constructed of cartilage. The entire endoskeleton is permanently 



Beyond question, most of the mesenchyme 

 comes from the mesoderm. The parietal and 



