THE DEVELOPMENT OF THE EXTERNAL FORM OF THE BODY. 223 



that similar statements have been made in connection with the Insecta, which 

 later research has not confirmed, and, moreover, the ontogeny of the Myriopoda 

 has not yet been exhaustively investigated, so that nothing definite is known as 

 to the relation of this furrow to the formation of the germ-layers.* 



The cleavage and the formation of the germ-layers in the Myriopoda has been 

 investigated by Metschnikoff in various Diplopoda (Strongylosoma, Polydesmus, 

 Polyxenus, and Julus, No. 11), as well as in the Chilopoda (Geophilus, No. 12) ; 

 the last-named form has also been investigated by Sograff (Nos. 19 and 20), 

 while Heathcote has re-investigated julus (No. 7). The earliest observations 

 are those of Metschxikoff, but these unfortunately were made without the 

 assistance of sections. Sograff's treatise being in Russian has remained to 

 a certain extent inaccessible to us, and the descriptions of Heathcote are 

 not very satisfactory. Besides these there is a treatise by Stecker on the 

 early development in the Myriopoda (No. 21). The results obtained by this 

 author are, as Balfour pointed out, in absolute contradiction to what is 

 otherwise known of the ontogeny of the Myriopoda. This author found no 

 free yolk, and observed a blastula giving rise to an invagination-gastrula witli a 

 wide archenteron. It has been conjectured by Sograff (No. 19), that Stecker 

 mistook Gastropod eggs for the eggs of Myriopoda, but this hardly appears 

 possible when we consider Stecker'f definite statements as to the species 

 examined by him and his description of their later stages. Further investiga- 

 tions of the genera examined by Stecker have led to totally different results, 

 which justify us in regarding Stecker's account as unreliable and inadmissible 

 in our account of the ontogeny of the Myriopoda. 



2. The Development of the External Form of the Body. 



A. Chilopoda. 



As has already been mentioned, the development of the blastoderm 

 is first completed on the ventral side of the egg, from whence it 

 spreads later to the dorsal side (Fig. 107 B). This ventral blastoderm 

 consists of small cells, and here the rudiment of the germ-band 

 appears, its cephalic lobes being first visible (Fig. 109, Id)'. Posteriorly 

 no differentiation can be observed, the rudiment of the germ-band 

 there fading away into the undifferentiated blastoderm, which has 

 not yet completely surrounded the egg. The rudiments of the 

 antennae appear early at the posterior boundary of this first 

 recognisable segment of the germ-band, the cephalic lobes (Fig. 

 109, B and C, at). The next segment to appear belongs to the 

 mandibles, and then follow the segments of the two pairs of 

 maxillae and the niaxillipedes (Fig. 110, md-mp). The limbs and 

 segments generally develop in regular order from before backward, 

 as can be clearly made out from Figs. 109 C, 110 and 111. 



According to Sograff, the anus forms very early in Geophilus 



* [Heymons (App. to Lit. on Myriopoda, No. III.) states that there is no 

 gastrula-groove in Scolopendra ; lie finds that the entoderm forms both from the 

 yolk-cells and by a budding oh of cells from all parts of the blastoderm. — Ed. ] 



