200 COMPARATIVE ANATOMY 



very slight traces of them are to be found in the full-grown 

 animal. 



The cavities of the gonadic pouches, however, are persistent 

 portions of the myocoele, being formed by the evagination of 

 the lower corners of the eleventh . to the thirty-seventh 

 myocceles. The details of the development of the gonads are 

 rather complex. A pocket-like projection from the lower end 

 of each of the named myocceles is pushed into the myoccele 

 next in front of it. The germ cells are formed from the 

 epithelium lining the pockets, and, as the latter increase in 

 size, each one thrusts the wall of the myocoele in which it 

 lies inwards towards the atrial cavity, thus giving rise to 

 a gonadic sac projecting into the atrium ; within this lies the 

 gonad derived from the wall of the myocoele next behind. 

 The whole process is described at length in Dr Willey's 

 Memoir on Amphioxus. 



The remarkable simplicity of the developmental processes 

 of Amphioxus bear witness to their primitive character. The 

 principal organs are formed as infoldings or outfoldings of 

 simple epithelial layers, and only at a comparatively late 

 period are the epithelial cells differentiated to form nervous, 

 muscular, notochordal, and other tissues. Thus the blastula, 

 by infolding, becomes a gastrula; outfoldings of the inner 

 layer of the gastrula give rise to the notochord and mesoblastic 

 somites. The central nervous system is formed by a sinking 

 in and subsequent folding up of a part of the outer layer ; the 

 atrial cavity, also, originates as a groove in the outer layer, 

 which afterwards is converted into a tube. 



The development of the higher vertebrates pursues a similar 

 course. In them, also, simple epithelial layers are formed, 

 from which the various tissues and organs of the body are 

 subsequently evolved. But in their case the processes are 

 so much complicated and obscured, partly by the presence 

 of food-yolk, partly by the development of embryonic organs, 

 and partly by the precocious differentiation of tissues and 

 organs, that they can hardly be interpreted without reference 

 to Amphioxus. 



Again, in the adult anatomy of this remarkable animal, 

 we find the most simple example of the vertebrate plan of 

 organisation. There is an endoskeleton, in the form of an 



