438 INVERTEBRATA CHAP. 



noticeable ill effects. It is readily obtained in great numbers at 

 the abattoirs. 



The eggs are contained in the two uteri. They do not commence 

 to develop until they have been laid, but once they have commenced 

 to develop they are wonderfully tenacious of life. If they are spread 

 on slides and attached to the slide with albumen fixative, and the 

 slides are then put into a moist chamber or even into weak formalin, 

 their vitality is quite unimpaired and they pursue an orderly course 

 of development. This can be suspended by exposing them to cold, 

 and when the temperature rises the development is resumed at the 

 point at which it left off. This circumstance makes the development 

 of Ascaris an extremely convenient subject to work at for a professor 

 burdened with professional duties ; when he has leisure the develop- 

 ment is allowed to proceed, when he is busy with other duties the 

 development is suspended by exposing the eggs to cold. 



A word or two on the adult anatomy might be in place here. 

 It is characteristic of most Nematoda to have two female organs, 

 which open by a pore situated on the mid-ventral line about one- 

 third the length of the body from the front end. Each of these 

 organs consists of a long tube. The distal end of this tube is 

 practically solid and contains large nuclei embedded in a mass of 

 cytoplasm, in which cell divisions are not obvious. As one proceeds 

 i'arther down the tube a cavity appears, and the cytoplasmic 

 territories of the various nuclei become delimited from each other 

 so that we can speak of a layer of cells surrounding a cavity. 



These two sections of the tube are known as the ovary. Below 

 this the ova become detached from the lining of the tube and lie 

 in its cavity. This portion is termed the oviduct. Here the 

 maturation divisions take place and the male cells meet and unite 

 with the female cells. Below this the tube widens and the eggs 

 become surrounded by their horny egg-shells, in which they 

 complete most of their development. This section is termed the 

 uterus. Finally there is an extremely short terminal piece common 

 to the two tubes and known as the vagina. 



The sexes are separate in Ascaris megalocephala as in the great 

 majority of ISTematoda. The male organ consists of a single tube 

 which, in a manner similar to the topography employed in the case 

 of the female organs, is mapped out into a testis, without cavity 

 and with undivided cytoplasm and numerous nuclei, a hollow vas 

 deferens, in which the male cells become detached and mature, and 

 a terminal vesicula seminalis in which the mature male cells are 

 stored up. This tube opens near the posterior end into an ectodermal 

 pit or atrium, the cloaca, into which the posterior end of the 

 alimentary canal also opens. 



Two horny spicules, inserted in ectodermal pockets of the cloaca at 

 the sides of the male genital opening and capable of being extruded by 

 special muscles, are used to distend the genital opening of the female 

 and effect fertilization. These are known as copulatory spicules. 



