EMBRYONIC DEVELOPMENT. 



67 



sides (rudiment of the mid-gut), which soon grows out posteriorly 

 as the rudiment of the intestine, and two lateral coelomic sacs 

 (rudiments of the middle germ -layer and the body-cavity) — Fig. 

 29 B. During the whole of larval life the alimentary canal remains 

 closed, the rudiments of the mouth and anus being wanting. The 

 coelomic sacs grow completely round the mid -gut, the walls of 

 the two sacs becoming applied later to form a dorsal and ventral 

 mesentery. 



The embryo now grows somewhat in length and becomes marked 

 off into two parts by a circular furrow. The anterior part of the 

 body is soon cut up by another circular furrow into two regions, 

 so that the animal now consists of three parts not quite of equal size 

 (Fig. 29 B).* These three parts have repeatedly been called 

 segments, but we shall see that they are 

 in no way true segments, and we shall 

 therefore name them the cephalic, the 

 thoracic, and the pedM regions. The latter, 

 denned by Kowalevsky as the caudal 

 segment, contains only the posterior pro- 

 longations of the coelomic sacs (Fig. 29 B), 

 the mid-gut belonging to the two anterior 

 regions. 



The cephalic region gives rise later to 

 the umbrella-like cephalic section of the 

 body which is surrounded by a ring of 

 cilia (Fig. 30), and carries at its apex four 

 symmetrically placed eye-spots, the dorsal 

 pair of which appears first. In the thoracic 

 region, a fold is soon found growing out 

 posteriorly (m) ; this fold, which at first 

 is circular and then becomes divided up 

 into a dorsal and a ventral lobe, must be 



regarded as the rudiment of the two mantle-lobes, and will here be 

 '•ailed the mantle-fold. It carries ventrally two pairs of provisional 

 tufts of setae (b), and almost completely encircles the pedal region 



* Authors differ with regard to the origin of the middle region of the body. 

 According to Hoyer (No. 7), it becomes cut off from the anterior section of 

 the embryo when the latter consists of two sections. This is confirmed by 

 Shipley's statements in connection with Argiope. and Lacaze-Duthieks' in 

 connection with Thecidium. Oeiilert and Deniker (No. 9), on the contrary, 

 agree with Kowalevsky in the view that " Le segment median s'est probablo- 

 nient forme par la division du segment caudal." It cannot be denied that this 

 is a point of some importance. 



Fig. 30.— Free-swimming larva of 

 Argiope (after Kowalevsky, 

 from Gegenbaur). b, setae ; 

 m, mid-gut ; m, mantle. 



