236 ELEMENTARY BIOLOGY. [CIIAP. 



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 blastomcres ; segmentations of /?, around the 

 nuclei. The furrows which mark them off do not 

 extend into the yolk-laden protoplasm. 



h. The same at the gasfrula 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 etidoderm; a small sac, derived from a by in- 

 vagination, completely enclosing the archentero?i. 



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 eti face. 



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

 thickened and expanded in front to form two 

 rounded proccphalic lobes. 



/3. The blastopore ; a small oval aperture, at the 

 extreme hinder end of a. 



y. The abdominal papilla; a median outgrowth of 

 tf, immediately in front of the blastopore. 



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



which the blastopore has disappeared. Treat as at^. 



