ORIGIN OF THE MESODERM 



459 



the notochord extends from the region of the anterior lip of the blasto- 

 pore, so it is assumed that this is also the case in birds. 



If the student will think of assumptions and incidents of this kind, 

 and note the manner in which hundreds of such assumptions and inci- 

 dents must be gathered from all angles and from hundreds of experi- 

 ments by hundreds of different investigators, to make such a. study as 

 embryology possible, he will obtain at least some slight appreciation of 

 what scientific investigation means and what scientific method means. 



In reading the literature of the subject, the student will note that 

 probably most writers insist that the notochord develops from the ento- 

 derm, though there are those who believe it comes from either of the 

 other two layers, and some even that it comes from all three. 



Fig. 266. 



B 



A. Surface view of Embryo at the Twenty-third Hour of Incubation. 

 A., anterior limit of head; AP., area pellucida ; AV., area vasculosa ; B., 

 border of mesoderm ; C.A., yolk crescent ; H., Hensen's node ; P., pro- 

 amnion ; PP., primitive streak ; PV., mesoblastic somites ; St., sinus ter- 

 minalis bounding the vascular area ; U., unsegmented mesoderm. I, region 

 where the medullary folds have almost met to form the medullary canal. 



B. Anterior part of the preceding figure more highly magnified to show 

 details. A, ectoderm of anterior end of head ; B, mesenchyme ; C, sub- 

 cephalic pocket: 1, region where the medullary folds will begin fusing to form 

 medullary canal; 2, margin of the anterior intestinal portal; 3 and 4, 

 posterior regions of medullary folds ; 5, lateral limits of head region ; 6, 

 border of foregut. (From Duval.) 



In all forms studied, however, the notochord is not seen to arise 

 from any definite layer, but it arises either at the same time the meso- 

 derm does (Fig. 267), or from the undifferentiated growth of cells about 

 the closed blastopore which gives rise to both entoderm and mesoderm. 



The notochord itself is a rod-shaped structure, circular in cross sec- 

 tion, extending headward from Hensen's node. 



THE NEURAL PLATE 



A thickening of the ectoderm at about eighteen hours' incubation 

 causes a greater density along each side of the notochord. This denser 

 area is several cells in thickness, and forms what is called the neural or 



