242 THE E.\r BRYOLOGY OF THE BLOW-FLY LN THE EGG. 



\ 



V\<... 21^. — /, An Optical Section of an lOmbryo in the stage represented by sections, 

 Figs 3 to 6, n. XIII. ; 2, A Diagram constructed from Tranverse Sections. The 

 parts surrounding tlie yelk are drawn larger than they really are, so that the yelk 

 is represented by too small a space : am, amnion ; / b, portion of the amnial 

 tube which becomes the primitive band ; / a, ):)riniitive archenteron : /, pro- 

 cephalic lobe ; s, serosa ; // /', ///', head-fold. The position of the future stomo- 

 dreum is represented by the di.itted line si. The shaded nuclei indicate cells 

 which occupy the median line. The arrow, j', shows the communication between 

 the folds of the serosa and the yelk ; the second arrow below this shows the 

 opening into the aninial lube in the cephalic region. 



Di:SCRIPTION' OF Pl.AlK XIII. 



The figures in this plate are all copied from Graber [114]. 

 Fn;. I. —A section of the egg of a Blow-fly, showing the segmenting morula within 



the yelk (a condition regarded by Graber as abnormal). 

 Fii;. 2. — The posterior pole of the blastoderm of the Elow-tly in the blastula stage, 



showing the polar cells. 

 Fic;s. 3 to 6. — Four sections of the ovum of the Blow-fly in the stage represented by 

 Fig. 3^- 



The position of these sections is indicated by dotted lines in Fig. 38. 

 Fi'i. 3 represents a section taken in the region indicated by line i. 

 Fn.. 4 represents a section in the region indicated by line 2. 

 Fn;. 5 represents a section immediately behind the head- fold, It f. 

 Fir,. 6 represents a section iii the region indicated by line 3. 

 The following refer to all the figures in which they occur : 

 <r, archenteron ; am, amnion ; It f, head-fold ; and mc, coelomic sac. 



