REPRODUCTION 85 



skeleton. It develops no structures morphologically similar to the heart, 

 kidneys, specialized sense organs, or paired appendages 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 Uving but make it conspicuously unlike any adult vertebrate. Never- 

 theless 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 development of Amphioxus to illustrate the main 

 features of the corresponding 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 

 occasions much difficulty and obscurity in the study and interpretation of 

 the processes. 



Organogenesis in the Vertebrates 



In the late embryo of Amphioxus the main lines of the body-plan of a 

 vertebrate are drawn. There are, however, some noteworthy differences 

 in the mode of origin of corresponding organs of Amphioxus and 

 vertebrates. 



Neural Tube. In Amphioxus the neural plate becomes detached from 

 the adjacent lateral ectoderm (Fig. 46) and transforms itself into a 

 tube not until after it has been covered by the lateral ectoderm. In 

 vertebrates a longitudinal folding of the neural plate and adjoining ecto- 

 derm occurs in such a way that the movement of the neural material 

 into a deep position, its conversion into a tube, and the covering of it by 

 lateral ectoderm take place simultaneously (Fig. 58). Not until the 

 tubular form is attained does the neural ectoderm of vertebrates become 

 detached from the overlying superficial ectoderm. Figure 59 shows, 

 in diagrammatic way, the characteristic appearance of a recently formed 

 neural tube with its neural crests, dorsolateral extensions 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 ganglia. Cells of the crest become 

 ganglion cells whence grow out nerve fibers which constitute the dorsal 

 sensory root of a spinal nerve. The fibers of the other constituent root of 

 a spinal nerve, the ventral motor root, grow out from cells within the 

 neural tube. Some cells of the neural crests migrate into various visceral 

 localities and give rise to ganglia and nerves of the autonomic system. 



The anterior region of the tube expands to form the brain. Three 

 enlargements, one behind another, the primary brain vesicles — fore-brain, 

 mid-brain and hind brain (Figs. 61, 63, 85) — characterize the cephalic 



