4(> GASTRULA THEORY 



that the cavity under the developing dorsal plate (embryonic axis) is roofed-in by the upper 

 wall of the canal and opens behind on to the surface of the primitive plate by the mouth of the 

 original invagination or blastopore. In some Beptilia this invaginated canal is wide, in others 

 narrow ; but it clearly corresponds to the solid protochordal process of mammals which is 

 tunnelled in its later stages, and in most forms only at its posterior end, by the notochordal 

 canal, which opens on the surface as the blastopore. It is a common experience in embryology 

 to find a developmental process modified, in the sense that a hollow rudiment is replaced by 

 a solid rudiment which is afterwards hollowed out. 



The space under the embryonal axis, formed in the manner described, may be taken, still 

 following this conception, as representing the dorsal part of the archenteron of Amphioxus 



FIG. 69. FOUR STAGES IN THE DEVELOPMENT OF AMPHIOXUS, ILLUSTRATING THE FORMATION OF THE 



GASTRULA. (Hatschek.) 



I. Spherical blastoderm ; the cells at the lower pole are larger than the others, and filled with 

 granules. 



II. Invagination of the lower pole producing a cupping of the vesicle. 



III. Completion of the invagination ; the blastoderm is now bilaminar, and forms a cup with 

 narrowed mouth, the blastopore, bl, and a double wall of ectoderm, ec, and entoderm, en. 



IV. The ovum is now elongated ; the cavity of the gastrula forms a primitive alimentary canal, the 

 orifice of which is the blastopore, which is directed dorsally. Extending from this along the dorsal 

 surface (right in the figure) a shallow groove is seen in optical section : this is the rudiment of the 

 nervous system. 



(fig. 71); and just as in that form, the entoderm of the roof gives origin to the notochord, while 

 the axial mesoderm arises from its lateral diverticula (ccelomic pouches). 



Van Beneden has shown that the floor of the notochordal canal persists for a time in the 

 bat; but in the Primates, and most other mammals, there is nothing to indicate the true 

 nature of the developmental phases, the whole series of phenomena being repeated in a still 

 more abbreviated form. Eternod ' has, however, observed in the early human blastoderm the 

 occurrence of scattered cells adhering to its under aspect, or to the lip of the neurenteric 

 canal, which he regards as traces of the floor of the archenteric sac or canal. 



1 Anat. Anzeiger, xvi. ; Compt. rend, de 1'Assoc. des Anat., 7 e reunion, Geneva, 1905; ibid. 

 & c Reunion, 1906 ; Bibliograph. Anat. xv. 



