42 T.H.MORGAN. 



of walking legs, and, moreover, the embryo has increased a great 

 deal in size. The first three pairs of appendages are essen- 

 tially as in the last stage. The fourth pair has now acquired its 

 adult number of segments, viz. eight. The two new segments 

 which have been added were interposed, the one between the 

 fifth and sixth (distal) of the preceding stage, and the other in 

 the middle of the fourth pair, in which we saw faint internal 

 traces of division in the last stage. The newly acquired fifth pair 

 has now six segments, three smaller proximal ones and three 

 distal. 



In the posterior part of the animal the undeveloped sixth pair 

 of appendages appears on each side of the body, and in between 

 the bases of these appendages appear the small 4 anlage ' of a 

 seventh pair. Five pairs of ventral ganglia are seen, the pos- 

 terior larger than in the last stage. The posterior part of the 

 abdomen lies at this stage still in the same plane as the rest of 

 the body. The diverticula of the digestive tract are in the first 

 appendages as in the last stage, the glands in the mandibles are 

 still present, and the fifth pair of ventral ganglia still retain their 

 invagination cavities. 



The * anlage' of the reproductive organs appears between the 

 first and second pairs of walking legs in the mid-dorsal wall of 

 the digestive tract, and is larger than in the last stage. In the 

 anterior part of the body there seems to be formed a large heart 

 cavity. 



After the next moult {stage 7), Fig. XVII, we see an important 

 change has taken place in the appendages. The third pair has 

 almost completely disappeared, and only two small stumps indi- 

 cate the point at which their appendage last appeared. The 

 first and second pairs of appendages are unchanged in structure. 

 I found one embryo of this age (7) having still the third pair of 

 appendages, and for some reason the larva was quite appreciably 

 larger than that given for this stage. Another pair of walking 

 legs has been acquired — the third — and each of these has six 

 segments. 



The second pair of walking legs (the fifth pair of appendages) 

 has now acquired its complete number of segments, eight, while 

 the first pair retains its previous number of eight. The new 

 segments which have appeared in the third pair of walking legs 



