REPRODUCTION 



8l 



As an introduction to the account of the development of organs in 

 vertebrates reference to Amphioxus will be helpful. 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 organs. 



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 pro- 



,NP ,NC 



Fig. 55. — AMPHIOXUS. Median longitudinal section of an embryo having 

 two mesodermal pouches, a stage approximately like that of the transverse section in 

 Fig. 46 E. The blastopore, roofed over by ectoderm, has become the neurenteric 

 canal. A, archenteron; EC, ectoderm; EN, endoderm; NC, endoderm destined to 

 become notochord; NE, neurenteric canal; NP, neural plate; P, neuropore. X350. 

 (Based on a figure by Hatschek.) 



cess of formation from the dorsal endoderm either side of the notochordal 

 fold, the pouches increasing in number by addition of new pouches succes- 

 sively from before backward (Figs. 46 and 47). 



In the course of further development the thickened ectodermal neural 

 plate becomes depressed slightly below the level of the neighboring lateral 

 ectoderm (Fig. 46, B-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 continuous 

 layer above the neural plate (Fig. 46E). 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. 46 F, G). The neural plate originally extends back to the blastopore. 

 The overarching process whereby the neural plate is covered proceeds 



