Reproduction 267 



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 middorsal ectoderm 

 has become delimited from the lateral ectoderm to form the neural 

 plate, the middorsal 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 on 

 either side of the notochordal fold, the pouches increasing in number 

 by addition of new pouches in successively more posterior positions 

 (Figs. 213, 214). 



In the course of further development, the thickened ectodermal 

 neural plate becomes depressed slightly below the level of the neigh- 

 boring lateral ectoderm (Fig. 213B-D). Along the line of demarcation 

 between neural plate and lateral ectoderm, separation occurs, follow- 

 ing 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. 213E). 

 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. 213F-G). 



The neural plate originally extends back to the blastopore. The 

 overarching 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. 222, a sagittal section of an embryo at this stage. Upon con- 

 version of the plate into a tube, the blastopore is left in communica- 

 tion with the lumen of the tube. At its anterior end, the closure of the 

 neural tube is delayed so that for a time its lumen is open to the ex- 

 terior by a small aperture, the neuropore. The extraordinary result of 

 these changes is an embryo whose prospective digestive cavity, still 

 devoid of definitive mouth and anus, communicates via the neuren- 

 teric canal (the former blastopore) with the hind end of the cavity of 

 the prospective spinal cord, and thence to the outside by the anterior 

 neuropore (Fig. 222, P). 



These relations, however, are merely temporary. Eventually neuro- 

 pore and neurenteric canal close. The definitive enteric apertures, 



