500 BERTRAM G. SMITH 



furrows at the equator on the antero-ventral side of the egg. 

 It usually disappears by the time the neural groove is well estab- 

 lished. 



In preserved material, the roof of the gastrocoele is consid- 

 erably paler than the remaining surface of the egg; during the 

 latter part of this stage the neural plate is usually differentiated 

 as a spatulate area extending from the dorsal lip of the blastopore 

 to a little distance in front of the upper pole of the vertical axis, 

 and distinguishable through the greater whiteness of its surface 

 (see especially figs. 223 and 224). 



The dorsal lip of the blastopore is not a perfect arc of a circle, 

 but is somewhat incurved on each side of a forward-extending 

 notch in the median line (figs. 138 to 145). 



At the time of its first appearance, the neural groove occurs 

 as a distinct furrow extending from the notch in the dorsal lip of 

 the blastopore forward in the median line for a distance of about 

 60 degrees; the anterior half is much broader and deeper than 

 the pog^erior half (fig. 140). In a slightly later stage, the neural 

 groove has extended to a total length of about 95 degrees but 

 is nowhere so deep as in the anterior half during the preceding 

 stage (fig. 141). It is now a rather shallow groove, narrow in 

 its posterior portion, wider and more broken by occasional deeper 

 depressions or fissures in its middle and anterior parts. These 

 early transverse furrows do not occur at very regular intervals, 

 and are probably only incidental to the process of infolding of 

 the tissues. 



A little later, the neural groove becomes decidedly deeper in 

 its middle portion (fig. 142). The change is not uniform through- 

 out this region, but instead there is a series of three or four large 

 pits or depressions at fairly regular intervals, giving a segmented 

 appearance to the groove. Sometimes this segmented condition 

 is very marked; it has been repeatedly observed in living mate- 

 rial. Gradually the segmented region, though less sharply marked 

 becomes more extensive than before (fig. 143) ; it is best seen in 

 living material viewed by transmitted light, when the neural 

 groove appears made up of a regular succession of alternate 

 light and dark areas. Shortly before the appearance of the neural 



