CHAPTER IV. 



THE CHANGES WJIICH TAKE J>I,ACE DURING THE SECOND 



DAY. 



1. The First Half of the Second Day. In attempting 

 to remove the blastoderm from an egg which has undergone 

 from 30 to 36 hours' incubation, the observer cannot fail. to 

 notice a marked change in the consistency of the blastodermic 

 structures. The excessive delicacy and softness of texture 

 which rendered the extraction of an 18 or 20 hours' blasto- 

 derm so difficult, has given place to a considerable amount of 

 firmness; the outlines of the embryo and its appendages are 

 much bolder and more distinct ; and the whole blastoderm 

 can be removed from the egg with much greater ease, 



In the embryo itself viewed from above one of the fea- 

 tures which first attracts attention is the progress in the 

 head-fold (Fig. 15). The upper limb or head has become 

 much more prominent, while the lower groove is not only 

 proportionately deeper, but is also being carried back beneath 

 the body of the embryo (Chap. n. 5.) 



2. The medullary folds are closing rapidly. In the region 

 of the head they have quite coalesced, a slight notch in the 

 middle line at the extreme front marking for some little time 

 their line of junction. The open medullary groove of the first 

 day has thus become converted into a tube, the neural canal, 

 closed in front, but as } 7 et open behind. For a brief period 

 the calibre of this tube is uniform throughout; but very 

 speedily the front end dilates into a small bulb, whose cavity 

 remains continuous with the rest of the neural canal, and 

 whose walls, like those of the canal, are formed of epiblast. 

 This bulb is known as the first cerebral vesicle, Fig. 14, FB, 



