236 ELEMENTARY BIOLOGY. [CHAP. 



deposited in conical masses whose bases look 

 outwardly. 



/}. The clear protoplasm; forming a superficial in- 

 vestment for the whole. Nuclei are regularly ar- 

 ranged within it. 



y. The blast07?ieres ; segmentations of /3, around the 

 nuclei. The furrows which mark them off do not 

 extend into the yolk-laden protoplasm. 



b. The same at the gastrula stage. Easily recognisable 

 by the large horse-shoe shaped blastopore. 



Make sections of the area of invagination, 

 through the long axis of the blastopore. Note — 



a. The ectoderm; now established as a superficial 

 layer investing the whole. 



/?. The endoderm; a small sac, derived from a by in- 

 vagination, completely enclosing the archenteron. 



c. The same, at a slightly later stage. Look for speci- 

 mens having on the surface a shoe-shaped white 

 patch (blastoderm), immediately in front of the 

 blastopore now greatly reduced in size. Preserve 

 as before, stain and examine en face. 



a. The blastoderm; shoe-shaped or heart-shaped, 

 thickened and expanded in front to form two 

 rounded procephalic lobes. 



/?. The blastopore; a small oval aperture, at the 

 extreme hinder end of a. 



y. The abdominal papilla; a median outgrowth of 

 a, immediately in front of the blastopore. 



d. The same, at a later stage. Look for specimens in 



which the blastopore has disappeared. Treat as ate. 



