ENTEROPNEUSTA 



387 



Ihich caused Bateson to believe them derh^ed from it, and not from the 

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

 le 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. 

 \^^m [Spengel found the union of the nascent gonads with the ectoderm less 

 ^^^Kntimate, 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 pa-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 



X 



C^' 



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

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

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

 ment of the nerve cord ; cu, cavity of the middle coelomic sac, which is already 

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

 lic layers. 



[differentiated in a pecaliar 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 

 leveloped 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 



