146 



PANTOPODA. 



ments of the ganglia a pit-like depression appears (Fig. 67 A and 

 B, e), round which the cells of the thickening show a regular 

 epitheloid arrangement (Morgan). An ectodermal depression thus 

 takes part in the formation of the ganglion. The invagination closes 

 later, but its cavity can still be recognised after the ganglion has 

 shifted inward, and has lost its connection with the ectoderm 

 (Fig. 68, e). 



"When the two anterior pairs of ganglia unite they appear as a single pair, in 

 which, however, there are four pits, which proves that this one pair is composed 

 of two. 



Morgan's statement as to the participation of ectodermal invaginations in 

 the formation of the ventral ganglia is so definite, that we do not seem justified 



a. 



in doubting this fact {cf. Figs. 67 

 and 68). He himself compares these 

 structures with the ventral organs of 

 Pcripatus (p. 189), and there is no 

 doubt a certain similarity between the 

 two ; but it must be pointed out that 

 the ventral organs are by no means 

 in such direct connection with the 

 ganglia as are the depressions in the 

 Pantopoda. 



A participation of an ectodermal 

 invagination similar to the above 

 in the formation of the brain cannot 

 be established, although it is just here 

 that we should expect it, when we 

 take into account the cerebral pits 

 in the Arachnida. 



The development of the ex- 

 ternal shape of the body is com- 

 pleted by the addition of the 

 missing appendages, the length- 

 ening of the embryo, and the 

 commencement of segmentation. 

 The first pair of limbs shifts 

 anteriorly and dorsally. At its 

 base, the proboscis or beak 

 appears to arise as an unpaired 

 anterior outgrowth of the body, carrying the mouth at its extremity. 

 At the posterior end of the body, the vestigial abdomen appears as 

 a small pointed appendage, at the end of which the anus forms. 



The first of the internal organs to claim attention is the enteron. 

 The entoderm has become arranged into an epithelium surrounding 

 the yolk-mass (Fig. 68, ent), and from this, diverticula, also filled 



Fig. 67. — A, embryo of Pallene cmpusa, seen 

 from the ventral side. B, part of a trans- 

 verse section through the same, to show 

 the paired depressions («) on the ventral 

 surface (after Morgan). I-VI, limbs; 6(7, 

 ventral chain of ganglia, the depressions 

 (e) being visible in the ganglia ; ect, ecto- 

 derm ; ent, entoderm ; mes, mesoderm. 



