103 
Histology and Embryology. By C. S. Minot. 
crop of Grrjllus cinereus, the European field cricket, there are sis 
teeth of very irregular shape, with many protuberances, but pre- 
senting, nevertheless, the general outline of a triangle, with the 
apex towards the middle. On each side of the projecting apex are 
two protruding points, at the base of which there is a bundle of 
stiff chitinous bristles. Between every two of these gigantic teeth 
there is a small ridge, which also has a hard cuticula. Further, 
the teeth are not attached along their whole base, but are partly 
drawn back, so that there is a space between the middle of the 
base and the muscular walls of the crop. The teeth form six 
regular, longitudinal rows, numbering each about twenty teeth. 
Their form varies according to the genera, and probably also 
according to the species. The walls of the crop are built up mainly 
of circular muscular fibres, which by their contraction drive the 
teeth towards the centre and so grind up the food of the cricket, 
thus performing a function which we are wont to think of as pro- 
perly belonging to the mouth. The study of the development of 
the teeth enabled Herr Wilde to ascertain that they are formed by 
underlying cells through a series of transformations of the cuticula, 
which appears at first as a simple membrane and then develops the 
secondary projections, which give the teeth their ultimate form. 
All these interesting discoveries could hardly have been made 
except by means of sections. 
The author has himself applied section-making to the study of 
the tracheae of insects.* It was found that the current descriptions 
in works on comparative anatomy and entomology were incorrect 
in several important particulars. The outside of the trachea is 
covered by a layer of flat polygonal cells, or, as it is called, a 
pavement epithelium. Thus in a longitudinal section of the main 
tracheal stem of the common water-beetle, Hydrophilus, the thin 
cells may be easily recognized by their nuclei. The epithelium 
secretes the enormously thick and complicated cuticula, which 
makes up the rest of the tracheal wall. The well-known spiral 
threads or filaments are part of this cuticula, and not distinct 
structures as was generally supposed. These threads run around 
the tubes and serve as elastic supports to keep the thin walls dis- 
tended ; they are more or less spiral, but instead of there being but 
one single thread, as is usually stated, there are four or five which 
end, after making a few turns around the tracheae, new ones arising 
to replace them. As the fibres run transversely, of course their 
cut ends only are seen in a longitudinal section. But these ends 
show that the filaments consist of a lighter outside, and a darker 
inside portion, which latter is round. The rest of the cuticula is 
divided into two layers, the inside one being slightly coloured by 
* Minot, “ Recherches histologiques sur les Trache'es de l’Hydrophilus piceus,” 
‘ Arch, do Physiol, normale et pathologique,’ se'r. 2, tom. iii. p. 1. 
