XIII 



PHYLUM CHORDATA 



57 



leaving a wide space around the latter. Segmentation is 

 complete, there being very little yolk : it begins by a 

 meridional cleft dividing the oosperm into two (B), and 

 followed by a second cleft, also meridional, at right angles to the 

 first (C, D). Next, an equatorial cleavage takes place, the embryo 

 coming to be formed of eight cells (E), of which the four be- 

 longing to the upper hemisphere, distinguished by the presence 

 of the polar cells, are smaller than the lower four. Further 

 meridional and ec^uatorial divisions take place, and the embryo 

 becomes a hlastula (I, K), enclosing a spacious blastocoele, and 

 having the cells on its lower pole larger than the rest. 



Invagination then takes place (Fig. 750, A), the lower pole of 

 the blastula becoming gradually pushed in nntil the whole lower 

 hemisphere is in complete contact wit>h the upper hemisphere 

 and the blastocoele obliterated (B). The gastrula thus formed 



Fig. 750.— Amphioxus lanceolatus. Three stages in the formation of the gastrula. 

 (From Korschelt and Ileidcr, after Hatschek.) 



is at first basin-shaped, having a very wide blastopore, but its 

 cavity gradually deepens, and the blastopore is reduced to a com- 

 paratively narrow aperture (C). At the same time the aspects of 

 the body are marked out : the dorsal surface becomes flattened, 

 the ventral convex ; the blastopore marks the posterior end and 

 is distinctly dorsal in position. Cilia are developed from the 

 ectoderm cells, and by their vibration cause the embryo to rotate 

 Avithin its membrane. 



The ectoderm cells forming the median portion of the flattened 

 dorsal surface now become differentiated and sink below the rest, 

 giving rise to the medullary plate (Fig. 751, A, mi^). The ordinary 

 ectoderm cells on each side of this plate rise up as a pair of 

 longitudinal mechdlary folds (hh), extend towards the middle line 

 and unite (B, hh), covering over the medullary plate. The latter 

 bends upwards at the sides so as to become trough-like instead 

 of flat (C), and, its two sides coming in contact with one another 

 above, the plate is converted into a tube, the neuron (Y), n), 

 enclosing a central canal, the neuroccelc, continued dorsally into 



