DEVELOPMENT OF THE EXTERNAL FORM OF THE BODY. 



343 



2. Development of the external form of the body. 



The germ-disc, whose form is now that of an elongated oval, and 

 the bilateral symmetry of which is marked by the remains of the 

 indistinct primitive groove, next becomes divided by a transverse 

 furrow into an anterior cephalic area and a posterior post -oral 

 thoraco-abdominal region. A second transverse furrow very soon 

 occurs behind the first, cutting off from the thoraco-abdominal region 

 the most anterior thoracic segment. This stage, in which the body 

 consists of a rounded cephalic portion, a still unsegmented posterior 

 region, and a trunk-segment intercalated between these two, strikingly 

 recalls a similar stage observed in Scorpio. The median furrow 

 extends anteriorly into the cephalic segment, while posteriorly 

 it is lost in the unsegmented region of the body. New thoracic 

 segments now successively separate from the posterior region, until 

 the number of free thoracic segments amounts to six. A similar 

 stage has been observed in the Araneae (Vol. iii., Fig. 25 A). 



FIG. 155. Two embryonic stages of Llmulus (after KINGSLEY). A, stage with primitive 

 groove. B, stage with rudiments of limbs. 1-6, the six thoracic limbs; a, anus; 7>, 

 primitive groove (blastopore) ; do, dorsal organ ; me, germ-disc with subjacent mesoderm- 

 layer ; m, mouth ; n, neural groove; a lt rudiment of the operculum (first abdominal limb). 



We now distinguish (Figs. 155 B and 162) a semicircular, anterior 

 cephalic region, six thoracic segments, and a posterior abdominal 

 region, also semicircular in outline. The rudiments of the limbs 

 (the chelicerae, l y and the five following pairs, 2-6) very soon appear 

 on the thoracic segments, at first as button-like prominences. The 

 smallest of these are the rudiments of the chelicerae, and each 

 subsequent pair is larger than the one in front of it. The oral 

 and anal apertures (m and a) are marked by ectodermal depressions. 

 The first of these lies in the cephalic region, and consequently in 

 front of the first pair of limbs (chelicerae), which belong to the 

 first thoracic segment (PACKARD, KINGSLEY). In later stages, the 



