516 



LOCV. 



[Vol. XI. 



is very clearly exhibited along the lateral margins of the neural 

 plate, extending from the unsegmented tip backwards, and, as 

 in Fig. 25, is continued in the embryonic rim to the points on 

 either side of the latter, where it is broken from the rest of the 

 blastoderm. While it is the lateral margins that are most 

 clearly divided into segments, in the trunk region the lines of 

 division may be traced inwards towards the median furrow. 

 This is probably due to the appearance of the mesodermic 



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Cut 3. — Twelve transverse sections of the embryo figured in the preceding cut. x about 30 

 diameters. The numbers refer to the positions of the sections in the series. 



somites in that region. Fig. 64 represents a horizontal section 

 of this embryo showing metameres in the ectoblast. 



Fig. 27 is in many respects similar to Fig. 26 ; it is slightly 

 older and has reached a length of 2 mm. and shows about five 

 mesodermic somites. Diverging furrows have appeared upon 

 the cephalic plate that include between them a wedge-shaped 

 central piece which terminates in the anterior unsegmented tip 

 before mentioned. The cephalic plate is thus separated into a 

 median and two lateral parts. It will also be noted in this figure 

 that the lateral margins are marked off from the rest of the 

 medullary plate by two furrows running lengthwise of the em- 

 bryo, so that the plate is bordered, as it were, by marginal 

 bands. The furrows are most distinct in the head region, but 



