232 THE EMBRYOLOGY OF THE BLOW-FLY IN THE EGG. 



blastocele forms only temporary structures — the echinopsedia 

 or ciliated embryo in the one, and the membranes the serosa 

 and amnion in the other. 



Centrolecithal Yelk Segmentation. — It is indubitable that cells, 

 or according to some nuclei, appear in the interior of the 

 yelk in Arthropods generally, and that these travel to its 

 periphery, and it is further believed that those which arrive 

 first at the surface of the yelk unite and form a single layer 

 of cells, the ectoblast, over its whole surface, and that others 

 which remain within the yelk form the hypoblast. The cells 

 which appear in the interior of the yelk are stellate and amoe- 

 boid, whilst the blastoderm itself consists of epithelial 

 elements. The manner in which these epithelial elements are 



DESCRH'TION of I'l.ATE XII. 



Embryological studies of various Insects after Graber. (All the figures except 

 Fig. 12 are from his last memoir, 118.) 



Kn;. I.— A median section of a six-days-old egg of the Burnet Moth (Zygoma 

 filifeiidula) ; showing the relations of the embryo and membranes to the yelk in 



the endolecithal type. 

 Fig. 2.— a transverse section of the egg of a field Cricket {Steiiohoihrtis variabilis) ; 



showing an early stage of an embryo of the epilecithal type. 

 Fig. 3. — An egg of .Stenobothius, showing a young embryo. 

 Fig. 4. — A more advanced egg of the same insect. 

 j.-k;. 5.— The isolated primitive band of a butterlly, Picris Cralitgi, from the egg on 



the fifth day. 

 I.-l(-,. 6.— The primitive band of the same on the sixth day. 

 J.-JO. 7. — A transverse section through the anterior part of the primitive band of the 



Hurnet Moth (Zygirita filipendiila), sliowing the amnion. 

 Fig. 8. — A surface view of the egg of a beetle, Lina inmula, on the second day. 

 Fio. 9. — The embryo of the same removed. 

 Fig. 10. — The head of the embryo of the butterfly, Pieris Cral,rgi, on the seventli 



day. 

 Fig. II. — Section through the first abdominal segment of a Stenobothrus embryo, 



showing the cuelomic sacs. (The right side is segmental the left interseg- 

 mental.) 

 Fig. 12. —.\ diagram of the egg of the Hlow-fly before the blastoderm appears on the 



surface of the yelk. 



'1 he dotted line shows the position and form of the blastoderm at an early 



stage of development. This diagram was constructed by Graber from a series of 



sections, and is given in his memoir on the fly embryo (114, page 140). 

 The following references apply to all the figures in which they occur : 

 a, archenteron, Mihi (proiloJuiii/i) ; am, amnion ; an, antenna ; g, ganglia; tinr, 

 mesanuiiboids of the yelk ; mc, mesoblast, or wall of the citlomic sac ; /a, para- 

 blastic-layer ; / *, primitive band j / r, proccphalic lobes; / i', primitive groove ; 

 s, serosa ; s g, salivary duct ; st, stomodanmi. 



