268 IXSECTA. 



perhaps represent the primitive condition, while in most Insects 

 there is a kind of abbreviation of development which causes some 

 of the cells to remain within the yolk from the first. Cf. in this 

 connection the formation of yolk-cells in the Crustacea (Vol. ii., 

 p. 144), Arachnida (pp. 43-45), and Myriopoda (p. 221). 



In the case of the Aphidae also, Will (No. 97) has maintained that the yolk- 

 cells arise exclusively through the immigration of cells from the blastoderm 

 during its formation. 



As a rule, all the blastoderm-cells are at first of the same shape and size. An 

 exception is afforded by the eggs of the Diptera, in which the so-called pole- 

 cells, to be described later (p. 352), which represent the early differentiation of 

 the genital rudiment, present us with elements which for a moment, indeed, 

 are incorporated in the blastoderm, but are distinguished by their size and their 

 contents from the blastoderm-cells {cf. Fig. 174 C, })z, p. 353, and Fig. 175 B, p, 

 p. 354). 



3. The Formation of the Embryonic Rudiment and the Embryonic 



Integuments. 

 A. General view of the Germ-band and the Germ-envelopes. 



The embryonic rudiment in the Insecta, as is often the case in the 

 Arthropoda, takes the form of a long band-like thickening, usually 

 extending along the ventral side of the egg, this being known as 

 the germ-band, embryonic band (Fig. 134 E). In most cases the 

 boundaries of the future body-segments are already indicated on 

 this band by consecutive transverse furrows. A cross section through 

 the germ-band of an insect (Fig. 133 B and C) shows it to be multi- 

 laminar. It consists* of an outer layer of cells, the ectoderm (ec), 



* We shall here therefore give the name "germ-band " to the whole embryonic 

 rudiment in contradistinction to the transitory portion of the egg, which com- 

 prises the food-yolk with its vitellophags and the embryonic envelopes. Such 

 a use of the term " germ-band " is universal in connection with the Arthropoda. 

 It should, however, be pointed out that in the Hirudinea (Vol. i., p. 321) this 

 term is used in another sense, and onl} 7 embraces a part of the embryonic 

 rudiment. Indeed, the expression "germ-band" is occasionally used as the 

 equivalent of "mesoderm-bands." 



t According to the published statements, we must assume that cellular 

 embryonic envelopes are not present in the Apterygogenea. They are said to be 

 wanting in the Poduridac (Ul.iaxix, No. 83). A cuticular larval integument, 

 such as is repeatedly found in other groups of Arthropoda (Arachnida and 

 Myriopoda, pp. 9, 58, 97), is said to form in this case. This may be provided 

 with prominences to assist in splitting the egg-integuments, and its presence 

 has been definitely proved by the observations of SoMMER (No. 76) and Lemoine 

 (No. 51). Indeed, it appears that the Podurid embryo passes through several 

 moults before batching. From this fact the absence of the amnion might be 

 concluded. GRASSI (No. 33) who observed a dorsal organ in Japyx which 

 occurs in the same way in the Poduridac, sees in this a proof of the presence 

 of the amnion. Since, however, in the Poduridac, this organ develops in the 

 earliest stages of the formation of the germ-band, it seems doubtful whether 

 we may compare it with the dorsal organs developing by the involution of the 

 serosa in the higher Insects (p. 304). We must therefore await further investi- 

 gations of these points. (See p. 304, and Hevmoks, No. XVI). 



