424 



half of the embryo, between the ectoderm and the yolk and with the 

 nuclei arranged over the greater part of it roughly in two layers. 



In a still later stage the second antennae and rostrum are con- 

 spicious. The mandibles are also present. The first antennae , which 

 are very small in the adult, cannot be identified yet. The other append- 

 ages are not yet free, so this may be called the nauplius stag. The 

 endoderm is beginning to separate from the mesoderm and forms a well 

 defined median ventral strand whose cells are arranged round a poten- 

 tial, — in places even an actual, — lumen. 



The line of very loose tissue marking the region of immigration of 

 the mesendoderm, and which underlies the whole endoderm (mesen- 

 teron) is very conspicious. The ventral blastodermic thickenings on each 

 side of this line have increased in depth. From the inner boundaries of 

 these thickenings will develop the two ventral nerve cords, while the 

 outer parts of the thickenings ultimately give rise to the body append- 

 ages. We now therefore have a deep ventral slit underlying the endo- 

 derm and filled by very loose tissue , indicating the line of past immi- 

 gration of the mesendoderm. 



This slit is open to the exterior ventrally, bounded on each side by 

 that part of the blastoderm which will give rise to the central nervous 

 system and covered in dorsally by the rudiment of the mesenteron (cf. 

 figure of next stage). 



In the next stage figured (fig. 4) the endoderm is more clearly dif- 

 ferentiated and marked off from the lateral sheets of mesoderm. In 

 front it passes into the stomodaeum and behind into the proctodaeum. 

 The stomodaeum has not yet got a lumen , but the proctodaeum is per- 

 forated. The lumen of the mesenteron is still mainly potential only. 



In most places the endoderm is still lying directly on the ectoderm, 

 but in places the mesoderm is beginning to get down between them. 

 Underneath the mesenteron we still find the slit filled with very loose 

 tissue. The edges of this slit, which will form the nerve cords, are now 

 differentiated off from the outer masses from which the appendages 

 are beginning to develop. 



Stomodaeum and Proctodaeum. 



The slit in the ventral blastoderm caused by immigration of the 

 mesendoderm into the blastocoele runs forward right up to the ventral 

 projection of the thickened ventral wall of the head which will form the 

 rostrum. As the mouth opens just behind the rostrum , it is clear that 

 it opens into the front end of this slit. 



The stomodaeum at first appears as a differentiation in the solid 

 mass of ectoderm forming the ventral wall of the head but whether this 



