423 



The ventral thickening of the blastoderm is now divided longitudi- 

 nally into symmetrical halves by the developing mesendoderm. This is 

 seen as a longitudinal median differentiation of the ventral blastoderm, 

 and also stretches up between the yolk and the blastoderm in the manner 

 show in the figure. The portion of the mesendoderm in the mid-ventral 

 line, embedded in thick and compact blastoderm, is of a very loose 

 texture in many places, doubtless indicating that migration inwards of 

 cells to form the lateral wings of mesoderm (as shown in the figure) has 

 been taking place. 



The section shows the very early appearing second antenna rudi- 

 ments at the side. The fat globule is now more closely approximated 

 to the ventral surface. It is to be noticed that the cuticle is no longer 



MESODERM 



MESENTERON 



opore\ Xrud - appendage 



PLUS 



RUD. NERVE CORP 



Fig. 4. Similar section to Fig. 3 at a later stage. 



present, having been cast in the interval between this stage and the last 

 one figured. 



In a somewhat later stage the process of inward migration of the 

 mesendoderm is complete , and we find it now entirely internal to the 

 blastoderm. The region from which the immigration took place is still 

 very conspicuous however and divides the blastoderm into right and left 

 halves by a mid-ventral line of extremely loose tissue. This line in trans- 

 verse section presents a most striking contrast to the compact many- 

 layered ectoderm bounding it on each side, and to the equally compact 

 mesendoderm lying across it dorsally and presents the appearance of a 

 slit through the whole thickness of the blastoderm, in which is lying a 

 little loose tissue. (Compare Fig. 4 of a slightly later stage.) 



The mesendoderm is in the form of a thin layer lining the ventral 



