uh. x FOWL— FIEST DAY 523 



are continuous anteriorly. The two medullary folds gradually extend 

 backwards and at the same time they become more prominent and arch 

 over towards one another until at about the end of the first day they 

 meet. It is to be noticed (Fig. 227, B) that the first meeting of the 

 medullary folds is some little distance back from their anterior 

 end, in about the position in which the division between mesence- 

 phalon and rhombencephalon will develop later. Towards their 

 anterior end the folds remain less prominent than they are farther 

 back with the result that the meeting of the two folds is here con- 

 siderably delayed. 



During these later hours of the first day important advances are 

 taking place in the development of the mesoderm. In the first place 

 it is to be noted that the anterior limit of this layer is gradually 

 extending forwards, encroaching more and more upon the proamnion 

 — the part of the blastoderm in front of the head fold which is still 

 two layered. In the second place the mesoderm becomes considerably 

 thickened and more compact in the region near the median line — 

 adjacent to the head process or notochord. This thickened portion of 

 the mesoderm becomes divided by transverse splits into a series of 

 blocks — the mesoderm segments — lying one behind the other (Fig. 227, 

 A and C, m.s). The first pair of splits to make their appearance are 

 placed obliquely, sloping outwards and backwards : they mark the 

 hind boundary of the first or most anterior segment. A little later 

 a pair of similar splits develop a little farther back forming the 

 hinder limit of the second segment, and so on, segment after segment 

 becoming separated off from the still continuous mesoderm lying 

 farther back. 



While this portion of the mesoderm is becoming segmented it is 

 at the same time becoming sharply marked off by its greater thickness 

 from the lateral, mesoderm lying farther out from the axis. Towards 

 the end of the first day a further important development takes place in 

 the mesoderm in as much as isolated splits appear in it parallel to its 

 surface and these gradually spread and finally become continuous so 

 as to divide the mesoderm- into the outer somatic layer next the 

 ectoderm and the inner splanchnic next the endoderm. The cavity 

 which has made its appearance between somatic and splanchnic layers 

 of mesoderm is the coelome. The portion lying within the myotome, 

 which soon becomes filled up by immigrant cells derived from its 

 wall, is the myocoele (Fig. 228, mc). The portion lying farther out, 

 in the lateral mesoderm, is the splauchnocoele (splc). The two layers 

 lying external to this cavity — the somatic mesoderm and the ectoderm 

 — constitute the somatopleure or body-wall : the corresponding layers 

 lying internal to the cavity — the splanchnic mesoderm and the endo- 

 derm — constitute the splanchnopleure or gut-wall. 



While the changes above described have been taking place the 

 blastoderm has constantly been increasing in area and by the end of 

 the first day it forms a cap, covering an extent of about 90° at the 

 upper pole of the egg. In the opaque area — the part of the blasto- 



