444 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



are somewhat larger than those of the cells of the ciliated bunds, and in carmine and 

 haematoxylin stain more deeply. On the rest of the ectodermal wall the contrast 

 between the ciliated bands and the intermediate zones is always more pronounced. 



In the region of the first band there occurs a A r ery active nuclear division, with 

 which the division of the cell bodies does not keep pace. As a result of this there 

 are usually found in a single cell body several nuclei, small but relatively rich in 

 chromatin, in different phases of division or in the resting stage. In the inter- 

 mediate zone the nuclei remain in the resting stage without undergoing further 

 division, and for that reason are relatively less numerous. They are also poorer 

 in chromatin, and in some no chromatin whatever can be demonstrated. These 

 nuclei without chromatin are very difficult to find. 



At the apex of the embryo, at the anterior end of the major axis, a small flat 

 cavity, the apical pit, appears. In this region the ectoderm is thickened and com- 

 posed of several layers. The apical pit often can be demonstrated in embryos of 

 from 40 to 50 hours, while it may be absent in others of equal age and equally 

 developed in other respects. 



The cells of the apical pit form the ciliated tuft of the free-swimming larva 

 which later becomes so prominent. They do not all give rise to cilia, however, for 

 a part of them migrate out of the epithelial layer and move deeper into the body. 

 This process is not confined to the cells of the apical pit, but occurs also in the 

 adjacent regions. 



The ectodermal elements are here now cut off from the more deeply lying 

 structures by a membrane just within which is a layer of fine fibrillse, apparently 

 nerve fibrillse of ectodermal origin. Within these fibrillse, resting upon the ends 

 of the prismatic cells, are pear-shaped cells possessing several processes. Apart 

 from their position they are differentiated from the mesoderm cells through con- 

 taining less yolk and having more finely pointed processes. These are evidently 

 ganglion cells of ectodermal origin. 



At first the fibrillae and the ganglion cells, indicating the embrj T onal nervous 

 system, are limited to the immediate vicinity of the apical pit, but later these extend 

 themselves further and further outward over the anterior end of the embryo and 

 along the ventral side. 



Some time after the appearance of the apical pit there appears just behind it 

 in the median ventral line a second almost circular flattening in the ectoderm, 

 which at first is very small. This deepens rapidly into a groove, which for a time 

 usually remains very small, but in many cases soon becomes very considerable. 

 During the later embryonal stages it continually broadens and eventually forms 

 the region of attachment of the free-swimming larva. 



The foremost ciliated band is ventrally interrupted by this fixation plane. 



In the thickened ventral surface there appears at about the eightieth hour 

 (earlier than the fixation plane), in the median line between the second and third 

 ciliated rings, an invagination which quickly becomes more extensive the vesti- 

 bular groove. The anterior portion of this, Avhich displaces anteriorly the second 

 ciliated band, is more depressed than the posterior, which lies near, or against, the 



