THE DEVELOPMENT OF AMPHIOXUS 37 



III. THE LARVAL PERIOD 



The duration of this period is roughly about three months, 

 during which the larva is free swimming but descends to deep 

 water. Development is very slow and consists largely in the 

 elaboration and modification or multiplication of structures 

 indicated at its beginning. Although up to this time devel- 

 opment has been simple and typical (primitive) in general, 

 now the anterior portion of the larva becomes highly modi- 

 fied through the development of characters not found in the 

 Craniata. These are, chiefly, the extensive asymmetry of the 

 pharyngeal and associated structures, and the development of 

 the atrium or peribranchial cavity both to be regarded as 

 special adaptations to the habits of the larva. The close of 

 the larval period is marked by an extensive metamorphosis 

 which consists largely in a " symmetrization" of the anterior 

 end. At the close of this period the larva is said to be at the 

 ''critical stage. 7 ' 



1. The Central Nervous System 



Very soon after hatching the walls of the spinal cord thicken, 

 constricting the neuroccel, and becoming differentiated into 

 three regions. These are (a), a thin epithelial layer the 

 ependyma, lining the neurocoel, (&), dorso-lateral and lateral 

 columns of nerve-cell bodies (gray substance] connecting re- 

 spectively with the dorsal and ventral spinal nerves, and (c) 

 ventral columns composed of nerve fibers (white substance). 



In the brain region the walls become comparatively thin 

 and the cavity dilates considerably (Fig. 13). At first the 

 brain lies beneath the epidermis, and around the neuropore 

 its wall is directly continuous with the superficial ectoderm. 

 When the dorsal fin appears in the mid-line the neuropore is 

 pushed to one side, usually the left. Gradually the brain 

 sinks away from the epidermis, drawing down the neuropore 

 into a funnel-shaped depression, and at the same time drawing 

 out the antero-dorsal wall of the brain into a conical elevation, 



