96 DEVELOPMENT OF ORGANS. 



Several varieties as to the mode of arrangement of the tuft of umbi- 

 lical vessels in the villi are described by Pro- 

 fessor Goodsir. The general rule is that one 

 umbilical vessel enters a villus, forms a simple, 

 more or less open loop, and passes out again with- 

 out dividing. Occasionally the vessel divides, 

 and its branches either pass out separate or anas- 

 tomose into a single trunk again. Sometimes 

 more than one vessel enters a single villus, which 

 either divide into other branches, or leave the 

 villus as they entered. Professor Goodsir con- 

 firms the fact observed by Professor Weber and 

 Mr. Dalrymple,! that the same vessel may enter 



and retire from two or more villi before it becomes continuous with 



a vein. 



OF THE DEVELOPMENT OF ORGANS.^ 

 Development of the vertebral column and cranium. 1 ^ 



Chorda dorsalis. A very minute account of the structure of the chorda 

 dorsalis in Amphibia and fishes, and of the metamorphoses undergone by 

 it, and by the surrounding cellular substance, in the formation of the 

 several parts of the vertebral system, has been furnished by Vogt, || 

 from observations made principally on the toad (Alytes obstetricans) on 

 Tritons, and on the salmon (Coregonus palsea). In the former of these 

 animals he observed, that at its earliest formation the chorda dorsalis con- 

 tains no cells, being composed of a clear gelatinous-looking substance, 

 with which are mingled, without any definite arrangement, numerous mole- 

 cular bodies and a few plates of stearin e. Shortly, numerous scattered 

 roundish cells appear, consisting of a fine membrane, enclosing a pel- 

 lucid gelatinous-looking fluid, and unprovided with a nucleus. They 

 are formed at first at the cephalic extremity of the chorda, but soon mul- 



* Fig. 18. "A diagram illustrating the arrangement of theplacental decidua. a. Parietal 

 decidua. b. A venous sinus passing obliquely through it by a valvular opening, c. A 

 curling artery passing in the same direction, d. The lining membrane of the maternal 

 vascular system (external membrane of villus) passing in from the artery and vein lining the 

 bag of the placenta and covering (e e) the foetal tufts, passing on to the latter by two 

 routes, first by their stems from the foetal side of the cavity, secondly by the terminal 

 and decidual bars (/"/") from the uterine side, and from one tuft to the other by the lateral 

 bar (#)." Throughout its whole course, except along the stems of the tuft, and the foetal 

 side of the placenta, this membrane is in contact with decidual cells. After Goodsir. 



t Miiller's Physiology, p. 1G05. 



t Section iii. chap i. p. 1609 of Miiller's Physiology. Page 1610. 



|| Entwick. der Geburtshelferkrote, pp. 41 ctseq.; and Histoire Naturelle des Poissons 

 d'eau douce, by M. Agassiz, t. i. : works already quoted from. 



