THE DEVELOPMENT OF AMPHIOXUS 27 



tened dorsal surface; length nearly twice the diameter, and the 

 diameter about equal to that of the egg. External surface 

 completely ciliated. Blastopore completely roofed, so that 

 the only external opening is the neuropore, opposite the 

 anterior margin of the first somite. Mesoderm folds are 

 formed throughout and two pairs of somites are constricted off 

 anteriorly. Notochord delimited but not developed. Archen- 

 teron open to the outside only by way of the roofed over 

 blastopore, through the space between neural plate and super- 

 ficial ectoderm, to neuropore. All organs and parts are still 

 formed of a single layer of epithelially arranged cells. 



During the remaining twenty-eight to sixty hours of the 

 embryonic period the chief developmental processes consist 

 in the elaboration of the rudiments already established rather 

 than the mapping out of new organs. 



B. THE EMBRYONIC PERIOD, FROM HATCHING TO THE FORMA- 

 TION OF THE MOUTH 



1. The Central Nervous System 



If not already accomplished, the neural plate is now rapidly 

 roofed over and the neuropore is carried to the anterior margin 

 of the first somite, where it remains throughout the larval 

 period. Next the neural plate is converted into a tubular 

 structure, the neural tube, through the depression of its me- 

 dian region and the accompanying rolling up and together 

 of its margins (Fig. 8). As in other Chordates this process 

 begins in the region of the first somite and extends thence 

 posteriorly and anteriorly. The original space between the 

 neural plate and the covering ectoderm becomes the cavity 

 of the neural tube or neurocoel (Fig. 9, B). The cilia of these 

 originally superficial cells now line the neurocoel and continue 

 to beat toward the posterior end of the tube. In the region of 

 the blastopore the neural plate remains in connection with the 

 continuous ectoderm and endoderm of that region, so that the 

 neurocoel does not close posteriorly, but leads directly into 

 the opening of the blastopore and so to the archenteron. This 



