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74 LOCV. [Vol. VI 1 1. 



especially noteworthy features. It consists of a semi-circular 

 fold of the embryonic rim, which curves around the end of the 

 median furrow and gradually passes into the sides of the body; 

 these in turn shade off into the rim of the blastoderm. 



The first observable change in the form of the head-end 

 arises from its becoming widened laterally, at the sam.e time 

 the anterior median portion of the head grows somewhat for- 

 wards, forming a blunt protruding tip. The head region be- 

 comes through these changes, divisable into a central portion 

 (Fig. I, T) and two lateral portions (Fig. i, L). The two 

 lateral portions expand rapidly sideways, overhanging the 

 blastoderm, and the form of the cephalic plate changes so that 

 its transverse axis becomes the major axis (Figs. 2 and 3). 

 The rest of the embryo is much narrower, and the whole 

 taken together may be spoken of as club-shaped. I apply the 

 term cephalic plate only to the much expanded anterior part 

 of the embryo, but this does not correspond with the brain 

 region, which extends further back into the narrower part of the 

 embryo. The posterior limit of the cephalic plate is marked 

 by two eminences (Fig. 3, x ; Fig. 4, 3) and behind these is the 

 region, before mentioned, in which the medullary folds are so 

 abruptly bent downwards. 



A tongue-like elevation, which is very noticeable in surface 

 views, soon makes its appearance in the median line (Fig. 2, 

 T). When it is first clearly outlined it extends from the an- 

 terior tip of the head backwards about one half the length of 

 the cephalic plate ; it is wedge-shaped in outline and its upper 

 surface is convex. This structure tends to mark off more 

 clearly the central portion of the cephalic plate ; it is separated 

 from the lateral portions by two distinct furrows. Although 

 it becomes less prominent in later stages, the tongue-like 

 process is readily distinguishable, until the head-folds finally 

 arch over it and hide it from view. 



The anterior part of the cephalic plate becomes the seat of 

 an infolding in which is formed both the infundibulum and the 

 optic vesicles. The first surface indication of this depression 

 is such as might be produced by pressing very lightly with 

 rounded dies on each side of the tongue-like process. Two 



