of a Sjpecies of Chironomus. 113 



By the growth of the vertical plates, the fore part of the 

 head is bent forwards. The vitelline streak situated in the 

 fore part of the head becomes diminished, and finally disap- 

 pears altogether. The inner embryonal envelope now covers 

 only the dorsal surface of the embryo, having removed by its 

 rupture, which commenced at the head, from the ventral sur- 

 face and the two extremities of the embryo. 



The constriction of the cephalic parts goes on, and at this 

 time also the vertical plates separate by a transverse furrow 

 from the caudal pad, by which means the head acquires a 

 distinctly limited form. Immediately after the limitation of 

 the head, the formation of the ventral segments commences, 

 first three and then the rest of the segments being formed. 



During the constriction of the cephalic parts, the embryo for 

 the second time revolves round its longitudinal axis, and again 

 by 180°, so that it gets to lie again in its original position; 

 i. e. its ventral surface moves again into the convex side of 

 the ovum. 



The further development of the head consists in the mandi- 

 bles occupying the place of the antennje, whilst the latter 

 move on to the sides of the fore part of the head. At this 

 time the intestinal tube is seen. 



After the constriction of the vertical plates from the germinal 

 pads is completed, the embryo draws itself together, which, 

 indeed, has really caused this constriction. The contraction 

 is indicated by the downward movement of the abdominal 

 extremity. At the same time commences the curvature of the 

 mandibles and the lateral movement of the first pair of max- 

 illaj, which become converted into the palpi ; the second pair 

 of maxillge, however, become united by their median margins. 

 On the lower surface of the thoracic segment a transverse 

 fold is formed ; and this is afterwards converted into the an- 

 terior legs. 



Finally, we see that the abdominal extremity has passed 

 entirely into the inferior pole of the ovum. The anal orifice, 

 formed previously by inversion, is now clearly visible. The 

 walls of the intestinal tube consist of a layer of large, oval 

 cells. 



In consequence of the constriction of the head and the 

 downward movement of the abdomen (or contraction of the 

 embryo) the vitelline mass, of course, passes outward and lies 

 immediately beneath the inner embryonal envelope. During 

 the whole contraction of the embryo, the lateral walls formed 

 by the rupture of the blastoderm grow into thin processes, 

 which gradually overgrow the vitelline mass, and finally close 

 the back of the embryo, as was quite correctly observed by 



