236 ESSENTIALS OF ZOOLOGY 



With the beginning of incubation, cell multiplication is 

 started afresh and the blastoderm is extended all round its 

 margin. It is now clear, as indeed is usually clear at the time 

 of laying, that the blastoderm is resolved into a central trans- 

 parent area, the area pellucida, and a marginal denser area, 

 the area opaca. This is due to the thinning of the endodermal 

 layer referred to above. The margin of the blastoderm makes 

 rapid progress, and it is of interest that the two layers are 

 separated in the process, the ectoderm advancing in front 

 of the endoderm. In about three days the sphere of yolk 

 has been half encompassed, and afterwards the two layers 

 advance more leisurely over the lower hemisphere. 



The blastoderm consists, as has been said, of two layers, 

 a thin ectoderm and an endoderm which is thin below the 

 pellucid area and is many-celled and thick all round the margin, 

 forming the area opaca. The central pellucid area is the 

 scene of changes which produce the embryo. Not long after 

 incubation is started the central ectoderm of the area 

 becomes thickened, marking the embryonic area ; and at 

 or near what proves to be the posterior end of the area 

 pellucida, the ectoderm commences to proliferate, forming a 

 medial ridge projecting downwards, and the ridge at once 

 liberates mesoderm cells which migrate outwards from it 

 between the ectoderm and the endoderm. The thickening 

 spreads forward in the pellucid area, and wings of mesoderm 

 are developed from it. Near the middle of the area pellucida 

 the ectodermal ridge comes to an end, and at this point a deeper 

 extension of the proliferation unites the two layers. In front 

 of the place of fusion proliferation still takes place, but from 

 the endoderm in the same longitudinal direction, and this 

 endodermal proliferation extends forwards to a point nearly 

 midway between the place of fusion and the anterior limits 

 of the thickened ectoderm which defines the embryonic area. 

 These changes occur almost simultaneously early in the first 

 day. It is not altogether a local proliferation, but the result 

 of growth in front of the region of fusion. 



The forward endodermal thickening which takes place in 

 a linear manner, marking the longitudinal axis of the embryo, 

 is the notochord ; and the thickening associated with the 



