ENTEROPNEUSTA 



387 



which caused Bateson to believe them derived from it, and not from the 

 mesoderm, as would seem more natural, especially since at this time 

 the mesodermal tissue is already found closely applied to the ectoderm. 

 However, Bateson states that the origin of the genital organs is as yet 

 not certainly determined. 



[Spengel found the union of the nascent gonads with the ectoderm less 

 intimate, and was inclined to refer their production to the mesenchyma 

 of the body cavity. The connection with the ectoderm is only secondary. 

 According to Morgan, they arise as iM'oliferations of the wall of the 

 coelomic sacs of the trunk, which at first have no connection with the 

 ectoderm. — K.] 



The central part of the nervous system arises, according 

 to Bateson, as follows : — A part of the cells of the deepest 

 layer of the ectoderm in the median line of the collar is 



Fig. 172. — Transverse section through the anterior part of the collar of a larva 

 of Balanoglossus Koxcalcvskii which is at about the stage of Fig. 168 B (p. 381) 

 (after Bateson). Above is seen the dorsal ciliated groove, d, intestine ; u, funda- 

 ment of the nerve cord ; Cn, cavity of the middle ccelomic sac, which is already 

 applied to the wall of the body and that of the intestine as somatic and splanch- 

 nic layers. 



differentiated in a peculiar way, and is finally detached from 

 the ectoderm along the entire length of the collar (Fig. 

 172 n). This process, moreover, is said to be accompanied 

 by a superficial depression of the ectoderm, which is notice- 

 able as a dorsal, longitudinal ciliated groove on the recently 

 developed collar of the young larva (Fig. 172, after Bateson). 

 Spengel also speaks of the development of the nervous 

 system as the result of a dorsal invagination in the middle 



