I02 COMPARATIVE ANATOMY 



duct splits longitudinally, one portion of it serving thereafter as the 

 Wolffian duct in connexion with the mesonephros while the other portion 

 acquires, by fusion of several pronephric nephrostomes, a wide anterior 

 opening into the coelom in the vicinity of the ovary. The oviduct 

 (Miillerian duct) thus formed has no connection with the kidney in the 

 adult. In other vertebrates, however, the oviduct develops as a fold of 

 peritoneum (Fig. 75, w) closely parallel to the Wolffian duct but inde- 

 pendent of it. The Miillerian duct, functional only in the female, develops 

 alike for a time in both male and female embryos but only vestiges of it 

 persist in the adult male. 



The Mesenchyme. Reference has been made (pages 94, 95) 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, becoming eventually more or less densely aggregated 

 in the spaces between the somite and neighboring organs or layers, plays 

 an important part in producing skeletal, connective and integumentary 

 tissues. This secondary mesoderm ("derm" implying a sheet or layer), 

 being usually not disposed in definite layers, is called mesenchyme. 

 But the somite is not the only source of mesenchyme. Quantities of it 

 are produced in all regions of the embryo and from various layers other 

 than the somites. 



Beyond question, most of the mesenchyme comes from the mesoderm. 

 The parietal and visceral layers of the hypomere are a prohfic source of it, 

 numerous 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 mass 

 of mesenchyme which accumulates between the enteric wall and the 

 adjacent layers of mesoderm. In fact, any mesodermal surface which 

 faces toward endoderm or ectoderm and any endodermal surface which 

 faces toward mesoderm is potentially a seat of mesenchyme formation. 

 The ectoderm plays a minor part but, especially in the anterior region of 

 the embryo, cells may be detached from the inner surface of the ectoderm. 

 Evidence has been found indicating that mesenchyme of ectodermal origin 

 participates in the development of the skeleton of the pharyngeal region. 



But no mesenchyme is produced at the coelomic surface of the meso- 

 derm or at the inner surface of the enteric endoderm. Mesenchyme in 

 the coelom or in the enteric cavity could have no constructive significance. 



Mesenchyme spreads from its place of origin and eventually is found 

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

 part which it plays is by no means secondary. Chief among its deriva- 

 tives are the following materials and structures. 



Comiective Tissue. Fibrous connective tissue is omnipresent in the 

 adult vertebrate. It invests, supports, connects, separates or cushions 



