CHAPTER XII. 

 THE ACCESSORY PARTS OF THE EMBRYO. 



SECTION 1. The Composition of the Embryo. 



Besides the blastoderm cells and their derivatives, the germ layers, 

 the salpa embryo contains a number of structures, which are partly 

 nutritive and partly protective, and the greatest peculiarity of its develop- 

 ment is the very complex way in which these various parts become 

 involved with each other. Their inter-relations are of such a character 

 that it is not easy to give a clear description of one of them without con- 

 tinual reference to the others, nor is any general description of the whole 

 possible until the reader has gained some knowledge of the history of 

 the various constituents. 



The composition of the embryo has been described in outline in the 

 chapter on the embryology of salpa, and I shall now describe each struc- 

 ture by itself, with such references to other parts as may be necessary. 

 This plan will involve much repetition, but it seems on the whole to be 

 the simplest and the least fatiguing to the reader, although I fear that 

 the history cannot be set forth in such a way as to be perfectly clear at 

 the first reading. 



The accessory structures are of two sorts : those which are derived 

 from the follicle = annexes de Tembryon, Barrois (6), p. 407; and the 

 foetal membranes. I shall describe them separately the foetal mem- 

 branes first. 



SECTION 2. The Foetal Membranes = Annexes extra-foztales, Barrois, 



(6), p. 467. 



I have nothing to add to the clear and simple account of the mode of 

 origin of the foetal membranes, and their anatomical relations, which 

 have been given by Salensky (5) and Barrois ; but my own observations 

 on the fate of these structures are, in many respects, so different from 

 the statements which are made by these authors, that it will be necessary 



