370 



EMBRYOLOGY 



noticeable as the fundament of the ventral ganglion (Fig. 

 162 B, vg). 



Many obscure points still exist regarding the further development. 

 In a species studied by Butschli two portions of the coelomic diverti- 

 cula lying in the head cavities are constricted off at an early period (Fig. 

 162 c. pv) ; the walls of these are said to be employed chiefly in the 

 formation of the musculature of the head. In the species studied by 0. 

 Hertwig, on the other hand, the formation of such paired head cavities 

 could not be recognized, for here in the course of the further development 

 the walls of the mid-gut and the coelomic sacs are so closely applied to 

 each other that these organs soon present only a slit-like lumen, which 

 finally disappears entirely. A solid, laterally compressed ectodermal 

 cord and two, likewise solid, lateral mesodermal masses, which contain 

 within them the genital products, can now be distinguished. All three 

 of the cords grow out backwards, not only in the region of the future 

 trunk, but also in the tail region, so that the latter also has an ento- 

 dermal fundament ; it is, however, smaller than that of the trunk region. 

 The rudimentary tail portion of the intestinal canal is subsequently em- 

 ployed in the formation of the sagittal septum separating the two caudal 

 cavities from each other ; here it atrophies without acquiring a lumen. 

 It has not as yet been observed either in what manner the transverse 

 septum between the trunk- and tail-cavities is formed, how the canal 

 opening is developed, or even how the efferent sexual ducts are produced. 

 Of interest is the great extension of the ventral ganglionic mass, which 

 remains united with the skin throughout life (Fig. 162 B, vg, and Fig. 



163 bg), extends in the young animal 

 along the ventral side and the lateral 

 parts of the entire trunk region, and 

 does not become relatively more re- 

 stricted until later. The transversely 

 striated fibres of the four longitudinal 

 muscle-bands are differentiated from 

 the cells of the somatic layer of the 

 lining of the coelom after the type of 

 epithelial musculature (Fig. 162 A, 

 so). The fins arise as simple evagi- 

 nations of the lateral parts of the 

 ectoderm, whereas the cuticular 

 skeleton found in them probably 

 arises as a secretion of this ecto- 

 dermal cell-layer at its base. In 

 later stages of development the two 

 coelomic sacs move in the trunk 

 region into close contact above and 

 below the intestinal canal, so that a 



Hies 



Fig. 163. — Traueverso section 

 through the trunk of Sagitta (after 

 O. Heetwig, from Lang's Le/irbucTi). 

 Ih, body cavity ; mes, mesentery of 

 the intestine ; md, mid-gut; Im, longi- 

 tudinal musculature ; bg, ventral 

 ganglion. 



