222 THE riROUSE IN HEALTH AND IN DISEASE 



part of the young worm appears to depend on the amount of water which is 

 imbibed from the outside, for if only such an amount of water be added to the 

 culture as is absolutely necessary to set the process of development in motion, and 

 the culture be then allowed to dry somewhat, it will be found that the embryo 

 is incapable of rupturing the egg shell. A slight collapse of the egg shell, 

 owing to an insufficiency of water, causes the death of the embryo at any 

 period of its growth. Hatching usually takes place in from thirty-six to forty- 

 eight hours after the egg passes out of the bird ; but in summer it may be 

 delayed for even as long as a month. 



When the embryo is hatched there seems little purpose in its early 

 movements. The cuticle, at first irregularly crinkled, gradually smoothens as 

 the parasite becomes saturated with water. The movements now appear to 

 gain in purpose, and very soon the little worm is actively moving about, 

 obviously in search of food. 



When newly hatched, the embryo measures 0'36 mm. in length, and 0'015 mm. 

 in greatest thickness (Fig. 22). The cuticle shows no regular striation. The body 

 Anatomi- i'"^ Cylindrical, tapering to a slender pointed tail in the last O'l of a 

 acters'o^f ™™- of its length. Anteriorly it maintains an almost uniform diameter 

 theembiyo. ^^ within 0"05 of the mouth, when it shows a slight and gradual narrow- 

 ing. The anterior extremity ends bluntly, and has a diameter of "007 5 mm., 

 presenting at its summit the small rounded opening of the mouth capsule. At 

 0'06 mm. from the tail the anal pore opens with but little external indication. 



Alimentary Canal. — Two faint parallel lines are seen running inwards for 

 a distance of O'Ol mm. from the oral pore. These are the walls of a cylindrical 

 mouth capsule, which later, with the growth of the worm, become much more 

 thickened and obvious. 



The oesophagus measures 0"1 mm. in total length, and is divided into two 

 portions. The anterior two-thirds is cigar-shaped, uniting by a narrow neck with 

 the posterior one-sixth which becomes bulbous. Surrounding this narrow neck are 

 a number of large refractile cells, forming the nerve ring of the central nervous 

 system. The oesophageal portion of the gut discharges into a long chyle intestine. 

 The epithelial cells of its wall lie at first almost in touch, and the lumen is 

 visible merely as a long, fine, wavy streak passing along the centre of the body. 

 The rectum is short, 0"01 mm. in length, and is cuticular. 



Upon the success of the embryo in obtaining a plentiful supply of food 

 depends almost wholly its future growth. If a freshly hatched embryo be 



