PROGRESS OF MICROSCOPICAL SCIENCE. 141 



such as canis and irritcms. It is more transjiarent, and permits the 

 various stages of development to be better observed. As the flea's egg 

 has been described by former observers, and especially by Leuckart, 

 M. Balbiani merely observes concerning its envelopes, that they con- 

 sist in a chorion and vitelline membrane, both very thin, transparent, 

 and colourless. The chorion is homogeneous, without sculpture, or 

 superficial reticulations. The rugose shell-like aspect its surface 

 presents does not arise from this membrane, as Leuckart thought, but 

 is caused by a coating the egg receives at the moment of its exjiul- 

 sion. The micropyle openings of the chorion are numerous, and are 

 found at the anterior as well as at the posterior pole. In these two 

 regions they are grouped in circular spaces, larger in the former, 

 where the micropyle holes number forty-five to fifty, while in the 

 latter there are only twenty-five to thirty. In the anterior group only 

 has M. Balbiani seen spermatic filaments engaged. One or two days 

 after laying, the formation of the embryon begins by a thickening of 

 a portion of the blastodern, in the form of a band, at first broad and 

 diffuse, but which gradually concentres on the ventral line of the egg. 

 The embryonary bandelet continues to grow at its posterior part, 

 whence it makes a fold which penetrates the vitellus, and bends roimd 

 to the dorsal, or opposite, side of the egg. This replicated, or caudal, 

 extremity of the embryo thus has for its origin a veritable invagina- 

 tion of the blastoderm at the posterior pole, while throughout the rest 

 of its length the embryo results from a local transformation of the 

 blastodermic vesicle, and consequently remains external to the vitellus. 

 This mode of formation of the embryo of the Pulicids presents a type 

 intermediate between that of the Dipters, in which the whole embryo 

 is exterior, and that of the Hemipters, in which it is chiefly, and some- 

 times entirely, formed at the expense of a portion of the blastoderm 

 invaginated in the vitellus. After remarking that the egg of the flea 

 is too small to make sections to exhibit the embryonic layers, and the 

 part they play in the process of development, M. Balbiani observes, 

 there is no difficulty in following the develojiment of the two mem- 

 branes which have received the names of the amnios and serous 

 envelope. With their formation, the first period of development 

 terminates, and at this early stage of evolution, the organ of repro- 

 duction is already visible in the form of a small cluster of clear 

 cells on the internal surface of the abdome^, immediately below 

 the posterior margin of the vitellus. No envelope surrounds this 

 mass of germinal cells, and the author formerly mentioned a similarly 

 precocious appearance of reproductive elements in Aphidians and 

 Lepidopters. The commencement of the second development period 

 is marked by the appearance of the rudiments of cephalic appendages 

 — antennee and mouth-organs — which last, by progress of evolution, 

 come to be organized as in maxillary or abrading insects (hrvyeurs). 

 We know that the larva of the flea feeds on solid matters, while the 

 perfect insect has a mouth adapted to suction. Another peculiarity 

 is the appearance of the rudiments of thoracic members, thouf;h the 

 larva is born in an apodal state. " This tendency to produce append- 

 ages like the legs of other insects, and which are destined to abort in 



