112 PRINCIPLES OF EMBRYOLOGY 



than a dozen cells, all of which have been derived from the Hght-grey 

 chorda-neural crescent. The lower three rows He inside the lip, and come 

 into the roof of the primitive gut; these give rise to the chorda, while the 

 remaining three rows stay on the outer surface and become the neural 

 plate. During the gastrulation, the arrangement of the cells in the rows is 

 profoundly altered; they slip between one another, so that the chorda 

 and neural plate, whose material was originally arranged in transverse 

 rows, become longitudinal strands running forward from the blastopore 

 lip. The effect of this is that the cells of the mesodermal crescent on the 

 other side of the egg have to move over towards the dorsal side to fill 

 the gap which would otherwise be left; and as the stretching of the dorsal 

 organs continues, the primitive gut becomes a tube with a strip of chorda 

 along its most dorsal part, a strip of mesoderm on each side of that, and 

 the main hollow of the tube lined with endoderm. 



As the elongation proceeds, the blastopore narrows like the mouth of a 

 laundry bag when the string is pulled tight. Before it is completely closed 

 in Amphioxus the ectoderm just above the ventral lip grows up in a curved 

 ridge, the sides of which rapidly come together, covering the still-open 

 blastopore from sight, and progress forwards above the neural plate, 

 like two flaps being drav^oi together by a zip-fastener up the mid-dorsal 

 line. Underneath these flaps, the neural plate continues to get longer and 

 narrower, and its centre sinks down to form a trough or groove; in the 

 most anterior end, indeed, it rolls up completely so as to produce a 

 neural tube, such as we shall find in the higher vertebrates. Meanwhile 

 important changes are beginning in the walls of the archenteron. These can 

 be most simply described by saying that the endoderm, which in a cross- 



(h) A dorsal view just after the completion of gastrulation. The blastopore 

 is at the bottom, but is covered by the two flaps of ectoderm which are 

 growing up over the neural plate, leaving a pear-shaped area of it visible. 

 The small circles arranged in a figure of eight are the nuclei in the endoderm 

 cells lining the archenteron. 



(»') Optical section at a slightly later stage, in the same orientation as g and 

 the previous drawings. Dorsal to the archenteron (arch.) the left row of 

 somites is seen lying above the notochord, and dorsal to that is the neural 

 plate, covered by the flap of ectoderm; blastop =hhstopore. 

 (j) Left view of 48-hour larva :^f. gut; n.t., neural system; ch, notochord. 

 {k) Section through the level of the second somite in Figure /, showing the 

 neural plate (dotted) beneath which are the notochord, and the somites 

 which are folding off from the walls of the archenteron. 

 (/) Section through same somites somewhat later. The neural plate is folding 

 into a groove ; the somites have separated from the gut-wall and from the 

 notochord, and the coelomic cavity has expanded within them. 

 (After Conklin 1932.) 



