20 OUTLINES OF CHORDATE DEVELOPMENT 



ficial cells, and during gastrulation these have developed motile 

 cilia, almost flagelliform, so that the gastrula is slowly rotated 

 within the egg membranes which still envelop it. The endo- 

 derm cells lining the archenteron are somewhat unlike and of 

 three distinct kinds, according to their origin. First there are 

 the deutoplasmic cells, which are the descendants of the original: 

 vegetal pole cells of the blastula, forming the greater part of the 

 floor of the archenteron; second, cells derived from the dorsal 

 margin of the blastopore which have been added to the endo- 

 derm through epiboly and inflection, forming a band along the 

 roof of the archenteron; and third, around the blastopore, a 

 rim of cells narrow ventrally but wider dorsally, formed also 

 through epiboly and inflection, from the ventral and lateral 

 margins of the blastopore. Thus the gastrula of Amphioxus, 

 although superficially resembling the simple type of invaginate 

 gastrula, such as that of many Ccelenterates, in reality is not like 

 that, for here epiboly plays an important part in its formation. 

 This is a leading characteristic of the gastrulas of the Chordates 

 in general, and it is important to recognize in Amphioxus this 

 method of gastrulation in its simplest and probably most 

 primitive form. If one but imagines that in Fig. 6, F, the 

 endoderm cells derived from the vegetal pole are multiplied and 

 filled with a great mass of yolk, the result will be not widely 

 unlike Fig. 32, E, of a section through the gastrula of the frog. 

 Development up to this stage has been so rapid that the com- 

 pletion of gastrulation occurs only six to seven hours after ferti- 

 lization. During the brief period between this stage and the 

 escape of the embryo from the egg membranes, elongation 

 continues slowly, chiefly through the rapid multiplication of 

 cells in a sort of "growth zone" around the blastopore. And 

 before the close of this period certain important structures are 

 marked out: these are, the central nervous system, the noto- 

 chord, and the mesoderm and coelom. 



4. The Formation of the Central Nervous System 



Along the dorsal flattened surface of the gastrula a median 

 strip of ectoderm cells becomes delimited from the adjacent cells 



