50 CHORD ATE ANATOMY 



but there is a high degree of inertia about it. The initiation of evolu- 

 tionary change is evidently not within the embryo itself. Its inertia is 

 such that it tends always to follow the old methods and it changes only 

 as it must. 



Organogenesis 



The earlier period of development is concerned with laying out the 

 building materials, the embryonic or "germ" layers. In the later and 

 longer period these layers are shaped into organs. The formation of the 

 central nervous organs and the notochord may begin, however, before 

 the mesoderm is fully established. Amphioxus, partly because it is so 

 small and partly because it is in so many respects primitive, affords 

 what may be regarded as a simplified and diagrammatic view of the early 

 relations of the organs in chordates. 



Organogenesis in Amphioxus 



In the preceding account of the early development of Amphioxus 

 the embryo has been followed to a stage where the mid-dorsal ectoderm 

 has become delimited from the lateral ectoderm to form the neural plate, 

 the mid-dorsal endoderm has given rise to a sharp thick upward fold which 

 is the prospective notochord, and paired mesodermal pouches are in 

 process of formation from the dorsal endoderm either side of the noto- 

 chordal fold, the pouches increasing in number by addition of new pouches 

 in successively more posterior positions. (Figs. 42 and 43) 



In the course of further development the thickened ectodermal 

 neural plate becomes depressed slightly below the level of the neighboring 

 lateral ectoderm (Fig. ^2B-D). Along the line of demarcation between 

 neural plate and lateral ectoderm separation occurs following which the 

 lateral ectoderm extends progressively over toward the median plane and 

 external to the neural plate. Eventually the edges of the right and left 

 sheets of ectoderm meet in the median plane and coalesce to form a con- 

 tinuous layer above the neural plate (Fig. ^2E), Meanwhile the neural 

 plate transforms itself into a tube by bending its lateral regions upward 

 and inward until the edges meet in the median plane where they become 

 joined. (Fig. 42F-G) 



The neural plate originally extends back to the blastopore. The 

 over-arching process whereby the neural plate is covered proceeds back- 

 ward and around the posterior margin of the blastopore. Thus neural 

 plate and blastopore come to lie under a common roof of ectoderm and 

 the blastopore, no longer opening directly to the exterior, opens into the 

 small space between the neural plate and its newly acquired ectodermal 

 roof. The resulting relation of layers and cavities are shown in Fig. 51, 

 a sagittal section of an embryo at this stage. Upon conversion of the 



