MYRIAPODA 



413 



soon cover the entire surface. Cells remaining within the yolk multiply 

 until up to 275 may be observed. These are the vitellophags. This 

 behavior is contrary to the observations of Silvestri (1898) on Pachyiulus, 

 where but few vitellophags remain in the yolk. 



On the fifth day of incubation the germ disk develops on the ventral 

 side of the egg in the form of paired sets of thickenings connected by a 

 longitudinal median swelling. An inner layer divided on the median 

 line then forms. Next there may be observed a two-lobed head, three 

 intermediate segments, and a diffuse tailpiece which passes over the 

 posterior pole. At first the germ disk gradually merges into the dorsal, 

 thinner part of the blastoderm, but later it becomes sharply delimited. 

 At the point where the median thickening joins the head segment a 

 thicker area is evident. Here a slight depression forms which later 

 becomes the stomodaeum. The germ disk now 

 elongates into a germ band, which at its period of 

 greatest elongation passes three-fourths around 

 the greater diameter of the egg. The germ band 

 also increases in width. The number of segments 

 then increases, the new metameres first appearing 

 by division of the head segment to form the 

 antennal segment. This is followed by the forma- 

 tion of maxillary, postmaxillary, and three rump 

 segments, each bearing later a pair of leg rudi- 

 ments. Although Lign'au states that the part 

 known as the "collar" is the tergite of the first 

 rump segment, Pflugf elder (1932), Heymons, and 

 Silvestri maintain that it represents the tergite of 

 the postmaxillary (labial) segment. 



Both the older and more recent workers are in agreement that the 

 mid-gut epithelium arises from a loosely formed strand of cells, first 

 solid but later wdth a lumen, extending through (not surrounding) the 

 yolk. Cholodkowsky (1895) states that this strand arises from anterior 

 and posterior entoderm rudiments which elongate and later fuse in the 

 middle. Lignau, however, says that the strand originates from an 

 anterior entoderm rudiment which elongates, pushes posteriorly in con- 

 tact with the blastoderm, and later separates from it. The ectodermal, 

 stomodaeal, and proctodaeal invaginations later unite with the entoder- 

 mal strand, then contract, straightening the line and thereby drawing the 

 thus completed alimentary canal into the yolk l>^ng dorsad of it. 



REMARKS ON DIPLOPODA IN GENERAL 



Silvestri (1898) and Robinson (1907) maintain that the postantennal 

 or premandibular intercalary segment is present in the Diplopoda, 



Fig. 370. — Polydes- 

 mus abchasius. Lateral 

 aspect of an egg contain- 

 ing about fifty peripheral 

 blastomeres. (From 

 Lignau.) 



