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House-Flies in Mesopotamia 



tinous envelope. The embryo itself consists of a thick-walled vesicle in 

 which two distinct poles are recognisable. At one pole, S, destined to 

 form the scolex, the wall is very thick, and its inner margin is difficult 

 to make out, as is indicated by the dotted line. The other pole is dis- 

 tinguished, not by any striking difference in its wall, but by the signs 

 of the beginning of a constriction at the points marked X. 



During life slow waves of contraction are continually passing over 

 the animal and consequently its shape may change very considerably. 

 Fig. 2 is intended to illustrate this point, Fig. 2 b being drawn from the 

 same specimen as Fig. 2 a, after an interval of three minutes. 



Stage II. This is figured in Figs. 1 b and 1 c. These two figures also 

 are drawn from a single specimen at different times, Fig. f c being drawn 

 one hour after Fig. 1 b. 



Fig. 2 a, b. Two drawings of another parasite, to show waves of contraction, taken with 

 an interval of three minutes between them. 



In the latter figure it will be noticed that the scolex-pole is little 

 different from that of Fig. 1 a, but the constriction observed in the latter 

 has now deepened so far as almost to separate off a flattened vesicle V. 

 The cyst itself has increased somewhat in size. 



After one hour we see, in Fig. 1 c, that the pole carrying the vesicle V 

 is little altered, but at the opposite pole a thick-walled, club-shaped 

 "head" is now marked out and folded over to one side. Remembering 

 what has already been said concerning the rhythmical contractions of 

 the animal it will be realised that it is difficult to say how far this 

 difference between the scolex-pole of Fig. f b and of Fig. 1 c was due to 

 these contractions or to an actual advance in development. 



Stage III. This is represented by two parasites from one fly drawn 

 in Fig. 3. 



The entire cyst has now increased considerably in size. The vesicle 



