12 T. H. MORGAN. 



At this stage they just touch on the middle line. Below this and 

 on the middle line is the thickening of the stomodseal invagina- 

 tion. In the center is a triangular-shaped cavity — the cavity 

 of the invagination. The base of the triangle is towards the 

 brain. On each side are the thickenings of the first pair of 

 appendages. In the lower part of these is a slight depression, 

 which indicates the line between the claw and the next segment 

 against which it works. Posterior to these various thickenings 

 appear two rows of opaque areas, the nerve ganglia. Three 

 pairs of these are seen in this figure, the first two being somewhat 

 smaller than the third. On each side of the last pair appear part 

 of the thickenings of the first pair of ambulatory legs (the fourth 

 pair of appendages). 



Fig. II is a continuation of the last figure, and shows the ven- 

 tral and posterior part of the same embryo. The upper pair of 

 ganglia are the second pair, and were seen in Fig. I, and the 

 second pair of this is the third of the last figure. On each side of 

 this third pair of ganglia are again the first pair of walking legs. 

 Two more pairs of ganglia follow this third. On each side of the 

 fourth and fifth pairs of ganglia appear the second and third 

 pairs of walking legs. An embryo at a stage later than the last 

 is shown by Fig. III. Here the embryo has elongated in an 

 antero-posterior direction, so that the figure shows only the 

 ventral side of the animal. Fig. IY is a dorsal view of an embryo 

 at the same stage. These figures show that the region about the 

 stomodseum has grown forward and has the opening of the stomo- 

 dseum at its distal end. This outgrowth is seen in the figures as 

 a forward prolongation of the embryo on the middle line. On 

 each side of it are seen the first pair of appendages, which have 

 also grown forward. 



On the ventral side of the embryo appear on each side of the 

 middle line five pairs of large ganglia (see Fig. III). On each 

 side of the last pairs appear the three pairs of walking legs, which 

 are longer than in the last figure, are bent, and stand out from 

 the surface of the embryo. Posterior to the last pair of ganglia 

 the embryo has a thickened mass of undifferentiated substance. 

 In a dorsal view (Fig. IY) we see the brain, and behind it the 

 large yolk mass of the embryo. The two halves of the brain 

 have come together, and each half is slightly bi-lobed. Around 



