OF OECANTHUS AND TELEAS. 239 



At this stage the amnion forms a complete covering over the surface of the embryo. 

 The thickened outer edges of the head fold are at this time continuous behind with the 

 outer edges of the germinal band, but they gradually grow in towards the median line 

 (pi. 18, fig. 5), and are at the same time bent forward towards the region of the future 

 mouth. The rounded angle made by the posterior end of the head fold is the first indica- 

 tion of appendages (the antennae). The anterior ends of the maxillary folds fuse and, 

 owing to the proportionately more rapid growth of the thoracic-abdominal than of the 

 cephalic region, the connecting portion of these folds lies posterior to the antennae and 

 forms a transverse elevation extending entirely across the embryo behind the cephalic 

 region, forming the anterior limit of the maxillary region. It will be noticed that it is an 

 unpaired structure. In the stage represented in pi. 18, fig. 5, the maxillary and thoracic 

 constrictions have become more sharply defined, since the folds have travelled to, and now 

 form the edges of, the embryo, but the strictly abdominal region has not kept pace in its 

 growth with the anterior regions. The embryo is now composed of four well marked 

 regions : cephalic, maxillary, thoracic and abdominal. 



The cross fold between the cephalic and maxillary regions grows fainter in the median 

 line, and there are gradually raised from the surface five pairs of protuberances (pi. 18, 

 figs. 8 and 9). These ten prominences which arise within the region of the maxillarv and 

 thoracic folds appear simultaneously. The forces producing them also cause a general 

 elevation of the lateral walls of the body above the surface of the germinal band. By 

 the elongation of the embryo that now takes place a space is left between the anten- 

 nal folds and the first pair of maxillary appendages, the floor of which is at first perfectly 

 level, but somewhat later is pushed up in the manner described for the five succeeding 

 pairs. The prominences arising in this space are first sharply defined on their posterior 

 borders. Whether this pair of prominences arises within the area marked off by the anterior 

 maxillary fold is still uncertain. At least they arise very close to, if not out of, the fold 

 itself. With its disappearance, the lateral thickenings of the maxillary and thoracic 

 regions become marked into segments by the appearance of four pairs of marginal notches, 

 so that the maxillary region is divided into two, and the thoracic into three, sharply 

 marked segments. These notches extend themselves across the embryo in the form of 



shallow furrows meeting in the median line. Later the abdomi- 

 nal region becomes segmented in the same manner but the 

 notches are never so distinct as in the anterior portion of the 

 embryo. 



The abdominal region has now increased in length until it is nearly 

 as long as the rest of the embryo ; its posterior part is broadened 

 Fig. 72. Fig. 23. m ^o a caudal enlargement, while its very tip is produced into a 



Fig. 22. ventral and side views sma ]i papilla. The head region meantime undergoes marked 



of an abnormally shaped embryo. ... 



x 40. changes, tor there occurs an invagination of the lateral ectoder- 



Fig 23. Abdominal region of ^ wal J in fr(mt of ^ bagftl en( j of ^ antenna l f ] t l. I n this 



an embryo of the same degree of 



development as the one shown in manner the limits of the aiitennal and brain regions are sharply 

 P i. is, fig. n. x 25. defined. While this invagination is progressing the lateral edge 



of the ectoderm in front of it becomes differentiated into a superficial, limited, crescentic 

 thickening and a deeper continuous layer. 



