Reproduction 271 



more anterior myotomes contain partially differentiated muscle-tissue 

 capable of feeble contraction. The animal is free-swimming but the 

 locomotor mechanism consists merely of long cilia produced by the 

 ectodermal layer. 



In its main features, this young Amphioxus is like a vertebrate. If 

 its true origin and nature were not known, it might reasonably be ex- 

 pected to proceed to develop directly into a typical vertebrate. But it 

 does not. It acquires no vertebral column; the notochord serves as 

 definitive axial skeleton. It develops no structures morphologically 

 similar to the heart, kidneys, specialized sense organs, or paired ap- 

 pendages of a vertebrate. Further, in later development it acquires, 

 especially in the head region, a variety of unique structures which 

 adapt the adult to its peculiar mode of living but make it conspicuously 

 unlike any adult vertebrate. Nevertheless, Amphioxus is "vertebrate" 

 in too many features to make it credible that they could have arisen 

 otherwise than in genetic relationship with those of the vertebrates. 

 Herein, then, lies in part the justification for describing the early de- 

 velopment of Amphioxus to illustrate the main features of the corre- 

 sponding stages of vertebrates. Further justification is derived, as 

 already stated, from the fact that the paucity of yolk in the egg of 

 Amphioxus relieves the embryo of the factor which introduces varying 

 degrees of complication into the development of vertebrates and occa- 

 sions much difficulty in the study and interpretation of the processes. 



Organogenesis in Vertebrates 



In the late embryo of Amphioxus, the main lines of the body plan 

 of a vertebrate are drawn. Brief statements concerning the embryonic 

 origin of the major organs of vertebrates follow. 



Neural Tube. In Amphioxus the neural plate becomes detached 

 from the adjacent lateral ectoderm (Fig. 213) and does not transform 

 itself into a tube until after it has been covered by the lateral ectoderm. 

 In vertebrates a longitudinal folding of the neural plate and adjoining 

 ectoderm occurs in such a way that the movement of the neural mate- 

 rial into a deep position, its conversion into a tube, and the covering 

 of it by lateral ectoderm take place simultaneously (Fig. 225). Not until 

 the tubular form is attained does the neural ectoderm of vertebrates 

 become detached from the overlying superficial ectoderm. Figure 226 

 shows, in a diagrammatic way, the characteristic appearance of a 

 recently formed neural tube with its neural crests, dorsolateral ex- 

 tensions of ectodermal material on each side of the tube. Later the 

 neural crest becomes detached from the tube, undergoes segmentation 

 corresponding to that of the myotomes, and gives rise to spinal gan- 

 glions (Fig. 151). Cells of the crest become ganglion-cells, whence grow 



