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



changes which occur between the series of sections repre- 

 sented by Graber, some of which I have copied in PI. XIII. , 

 and his series (Figs. 93 to 112) in which the primitive band 

 is already developed. My own observations do something 

 to bridge over the interval (PI. XIV., Fig. i), but are far from 

 complete. As the changes occur between the third and fourth 

 hour, according to Graber, they must be very rapid ; and as, 

 in my experience, the rate at which development in the &^^ 

 progresses is determined by temperature, it is by no means 

 easy to obtain sections in the exact stage needed for the obser- 

 vation. 



In Fig. 38, /, I have represented an ovum removed from the 

 shell about four hours after impregnation. Until I saw 

 Graber's figures I was at a loss to interpret the appearances 

 exhibited, but by the construction given beneath it (Fig. 38, 2) 

 this is no longer difficult. The great fold of the serosa, hf, h /', 

 is perhaps the ' b'altenblatt ' of Weismann [2], about which so 

 much uncertainty exists. I have never, however, seen it ex- 

 tending so far back as Weismann represents the ' Faltenblatt ' ; 

 neither is it developed, as Weismann believed, from a head and 

 tail fold. It is subsequently obliterated by the shortening of 

 the primitive band. 



The diagrammatic construction (Fig. 38, 2) exhibits a suggest- 

 ive similarity to a Lamellibranch, or an Ascidian, which will be 

 obvious to the morphologist. The great head-fold of the serosa 

 resembles a mantle, and the amniotic tube is suggestive of the 

 post-branchial chamber and atrium. The future position of 

 the ventral cord and dorsal vessel of the insect both correspond 

 with those of the nervous system and heart of the Lamelli- 

 branch. Moreover, it will be remembered that, whilst the 

 greater part of the cellular wall of the blastocele only forms 

 temporary structures in the Echinoderm and the Insect, the 

 Avhole becomes part of the body of the Lamellibranch, just as 

 the whole forms part of the insect embryo in the stage repre- 

 sented by my diagram. As I have no facts to guide me, 

 except the similarity of the form and disposition of the parts, I 

 am unable to pursue this subject further, and must leave the 



