226 MANUAL OF ANIMAL BIOLOGY 



vated to form the medullary folds. A little later the two folds, 

 running almost the length of the body, meet above the neural 

 groove in the midline and fuse, and thus a dorsal neural tube 

 of ectoderm is formed. From this ectodermal neural tube the 

 differentiated brain and spinal cord later develop. (W. f. 174, /.) 



3. Later Development 



Synchronously with the formation of the central nervous system 

 from the ectoderm, as just described, the embryo begins to lose 

 the spherical shape of the original egg and to elongate in an antero- 

 posterior axis so that the definite body regions can be identified. 

 Anteriorly the head is indicated, and just back of it on each side 

 of the body, the ectoderm becomes thickened and modified to form 

 the elevated gill arches through which lateral openings into the 

 pharynx (gill slits) later appear. Anterior to the gill region, a 

 swelling can be seen on the side of the head, which is the beginning 

 of certain sense organs. The ventral and posterior regions of the 

 body are still very heavily loaded with yolk material at this time. 

 Projecting from the extreme posterior end, on the dorsal side of 

 the embryo, is the tail bud which grows posteriorly to form the 

 tail region. A depression on the anterior ventral surface of the 

 head indicates the position of the future mouth, and a similar 

 depression at the posterior end of the body, near the original blasto- 

 pore, marks the future position of the anus. Just posterior to the 

 mouth depression, a crescent-shaped area indicates the ventral 

 sucker. During all the changes thus far, the animal is not feeding, 

 but is getting its nourishment from the utilization of the food 

 materials which were stored in the vegetal pole of the egg. (W. f. 

 174, J.) 



The microscopic examination of a longitudinal section, in a 

 sagittal plane, through an embryo in the 'tail-bud' stage shows the 

 following condition. There is an outer covering of ectoderm cells. 

 Lying ventral to the ectoderm, on the dorsal side, is the neural tube, 

 just formed by the fusion of a portion of the dorsal ectoderm. The 

 neural tube runs the entire length of the embryonic body and, at 

 this stage, shows very little differentiation. At the anterior end of 

 the embryo it curves ventrally, and the closed end lies in the ventral 

 half of the body. The curved, anterior end of this tube differenti- 

 ates into the fore-brain and the mid-brain. The hind-brain 

 develops from the region of the neural tube lying just posterior. 



