XIII 



PHYLUM CHORDATA 



51 



from the oosperm leaving a wide space around the latter Seg- 

 mentation is complete, there being very little yolk- it begins 

 by a meridional cleft dividing the oosperm into two (B) and 

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

 A ext, 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 

 the polar cell, are smaller than the lower four Further 

 meridional and equatorial divisions take place, and the embryo 

 becomes a Uastula (I, K), enclosing a spacious blastoccele, and 



:he cells on its lower pole larger than the rest 



Imagination then takes place (Fig. 705, A), the lower pole of 



blastula becoming gradually pushed in until the whole lower 



hemisphere is in complete contact with the upper hemisphere 



blastocoele obliterated (B). The gasfnila thus formed 



C 



^ZtJQjCVVggf 



IG. ,03. Amphioxus lanceolatus. Three stages in the forma tim n f tr, 



(From Korschelt and Heider, 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 

 body are marked out : the dorsal surface becomes flattened 

 ;he ventral convex ; the blastopore marks the posterior end and 

 distinctly dorsal in position. Cilia are developed from the 

 ectoderm cells and by their vibration cause the embryo to rotate 

 within 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. 706, A, mp). The ordinary 

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

 longitudinal medullary folds (Kb), extend towards the middle line 

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



Tfl , /K^ at the SideS S as to become trough-like instead 

 (UJ, and, its two sides coming in contact with one another 



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



E 2 



