74 



GENERAL SKETCH OF 



As the rim extends over the yolk the embryonic shield 

 lengthens, the keel thickens, and soon a spathulate flatten- 

 ing of the central or nervous layer is noticed anteriorly, that 

 is, at the opposite end from the aperture of the blastoderm 

 (the blastopore), the spathulate process trending posteriorly to 

 the margin of the aperture. Then differentiation of the spa- 

 thulate region (Fig. 23) occurs by the definition of the optic 

 vesicles from the solid cellular margin (Fig. 24). Very soon 

 after the formation of these organs traces of the ears appear 



FIG. 23. Egg of Whiting with 

 spathulate neurochord. 



FIG. 24. Egg of Cod with 

 embryo. Lateral view. 



more posteriorly, as elongated or elliptical thickenings of the 

 sensory layer of the epiblast, and a large fissure or chink develops 

 in the centre. 



To return to the growing blastoderm as it covers the yolk, 

 it is found that the thickened margin or rim bounds an aperture 

 called the blastopore by embryologists, this aperture attaining its 

 maximum when the margin of the blastoderm has reached the 

 equator, and thereafter decreasing in calibre as the rim advances 

 towards the vegetal pole of the yolk (Fig. 23) ; the latter indeed 

 in certain views bulges from the rim like an india-rubber ball 

 pressed in the fingers. The embryonic rim is used up in 

 the formation of the embryo. 



The aperture or blastopore then becomes contracted, 

 marked with radial lines or furrows, and finally closes. 



In eggs with oil-globules it is at this time that the globule 

 becomes surrounded by the protoplasmic periblast-layer which 

 envelopes the yolk. Only in one instance, and that probably 

 abnormal, has the globule been found freely moveable in the 



