318 A. T. MASTERMAN. 



rudiments into the median line from their very inception, 

 though even in Amphioxus the organ is formed by the 

 " dovetailing^^ together of two lateral rows of cells. 



I am indebted to Dr. Gadow for calling my attention to the 

 work of Mitsukuri (15a) on the ontogeny of Emys, in which 

 he has described paired rudiments of the notochord, which he 

 styles ^Miemichords/' and which fuse together in the median 

 dorsal line to form the notochord. The observation is signi- 

 ficant in the light of this discussion.^ 



There is one other possible interpretation of these " noto- 

 chords'^ of Actinotrocha, namely, that of a pair of abortive 

 gill-slits. In the light of the work on Cephalodiscus which 

 follows, it will be seen that this is perhaps a tenable hypo- 

 thesis, but it will be discussed under that head. 



The significance of the ventral chordoid tissue will be 

 referred to later. Assuming, then, that these organs may be re- 

 garded as primitive paired rudiments of the Chordate notochord, 

 I have depicted for the further comparison of the two types 

 one of these notochords in fig. 49, as bent into the mid-dorsal 

 line, in order to have it included in a median sagittal section. 



In fig. 48 is seen a similar section of Balanoglossus, com- 

 piled, in a semi-diagrammatic way, from the works of Spengel 

 (19a), Kohler (12a), and Morgan (17). A comparison of the 

 two will show the way in which it is here suggested that the 

 notochord of Balanoglossus has moved forwards. In doing 

 this it has come into peculiar relation with the pre-oral 

 coelora, which will be referred to again. 



We have in Balanoglossus the ''proboscis vesicle '^ of 

 Morgan (17) (''sac of proboscis gland ^^ of Bateson[l] and Kohler 

 [12a], Herz-blase" of Spengel [19a]), with its relationship to the 

 front end of the dorsal blood-sinus and to the pre-oral coelora, 

 and in Actinotrocha the subneural gland with precisely 

 similar relationships to these parts. This is even more clearly 



1 Davidoff (7a) lias shown that iu ceitaia Tiinicata the notochord and 

 nervous system arise from paired rudiments, and Brooks (lb) finds a similar 

 origin for tlie eieoblast of Sal pa. These facts may have more phyletic 

 significance than Brooks is inclined to allow. 



