History of the Nematode Worms. 451: 
are developed in the body of the mother like those of Ascaris 
acuminata, and usually pass into the stomach of the host still in 
the egg-capsule, in order there, after boring through the latter, 
to collect gradually in many hundreds in the rectum, in the form 
of small rapidly-moving vermicles; they present in their ex- 
ternal structure a great agreement with the Rhabditiform young 
states of other species. They have a rather stout form, and a 
length of 0:4 millim. The tail is short and pointed, and the 
anterior, buccal extremity is furnished with three small cuticular 
papille, which I have not met with elsewhere among the allied 
forms. The structure of the commencement of the genital or- 
gans is still more divergent. Whilst this is elsewhere (except 
in the Trichine, the embryos of which are peculiar in many 
other respects) always developed in the form of a small and 
nearly homogeneous clear corpuscle (0°02 millim. m length, 
with usually a simple nucleus), which shimmers through the 
ventral wall of the embryos about the middle of the chyle- 
intestine, it appears in the youngest forms of A. nigrovenosa as 
a considerable sac, of 0°08 millim. in length and 0-012 millim. 
in thickness, enclosing numerous distinctly recognizable cells 
with vesiculiform nucleus and nucleolus (cell=0-007 millim., 
nucleus = 0:0043 millim.). In previous grades of development 
this sac is recognized as an aggregation of embryonal cells, 
which separates from the other elements of the body of the em- 
bryo, and only changes in this respect, that the cells lose their 
previous coarsely granular texture, and acquire a more transpa- 
rent appearance. 
When the embryos are removed from the rectum of the frog 
(or even from the body of the mother) and kept in moist earth, 
they begin not only to grow rapidly, but also to develope their 
sexual organs; so that in a short time we have, instead of the 
previous young forms, sexually mature male and female animals. 
The duration of this period of development depends on external 
circumstances, especially the surrounding temperature. In the 
height of summer a single day sometimes suffices, whilst in 
winter it is a week or more (in the case of embryos taken from 
the body of the mother even as much as twelve days) before 
the animals arrive at sexual maturity. 
The sexual differentiation commences before the middle of 
this developmental period. It is introduced by a change of skin, 
after which the male individuals are to be distinguished by the 
shorter and blunter form of the caudal extremity, from the 
tensions raised by him in connexion with these investigations (Miiller’s 
Archiv, 1865, Heft 4) I refer to the next part of that journal, and the 
communication published in it by me. ; 
29* 
