38 



LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



extending along each side of the body axis from anterior to posterior end, and is 

 never composed of a series of cavities as in Amphioxus. The end result is the 

 same as in Amphioxus, although arrived at in a different way. 



The neural tube is formed from a pair of folds, the neural folds, which appear 

 in the median dorsal region, and fuse together to produce a tube. The notochord 

 is separated off as a solid cord of cells from the roof of the archenteron {Amphibia, 

 reptiles) or else arises from the somewhat indefinite region where the mesodermal 

 sheets originate (birds). 



The processes will be grasped more clearly by reference to Figure 7, also to 

 the figures in K, page 11; P and H, page 121; W, page 67. Obtain a mounted 



g ,h 



Fig. 7. — Diagrams to show the formation of the neural tube, notochord, mesoderm, and coelom 

 in vertebrates, based on Amphibia. Cross-sections. A, differentiation of the notochord/ in the roof 

 of the entoderm; mesodermal plates b spreading ventrally. B, neural folds h rising at the sides of the 

 medullary plate g; notochord / separated from the entoderm; mesodermal plates b extended farther 

 ventrally and developing a central cavity, the coelom i. C, neural folds h nearly closed to form the 

 neural tube j; mesodermal plates have reached the midventral line; coelomic split i has extended 

 ventrally. a, ectoderm; b, mesoderm; c, entoderm or archenteron; d, somatic mesoderm; e, splanch- 

 nic mesoderm; /, notochord; g, medullary plate; h, neural fold; i, coelom; j, neural tube. (A from 

 Hertwig-Mark's Textbook of the Embryology of II an and Mammals, courtesy of the Macmillan Company.) 



cross-section through an amphibian embryo at the stage of the formation of 

 the mesoderm, and examine under the low power. The section is oval in form; 

 it is in most cases still surrounded by the delicate egg membrane. The outer 

 layer of the embryo is the ectoderm, relatively thin and of the same width over 

 the whole surface. In the median dorsal line the ectoderm is producing or has 

 already produced the neural tube. In the former case the ectoderm exhibits a 

 pair of neural folds inclosing a thick plate of ectoderm between them. In the 

 latter case the folds have fused across in the median line, forming a tube, the 

 neural tube, which is the oval hollow mass in the median dorsal line, just beneath 

 the ectoderm. The greater part of the section is occupied by the archenteron or 

 primitive intestine, composed of entoderm. The archenteron has a thin dorsal 

 wall, a thick ventral wall, whose cells contain yolk, and incloses the relatively 

 small gastrocoel, which occupies its dorsal part. In the median dorsal region of 



