1897—98.] Dr Master man on At chimeric Segmentation. 299 
opening to the exterior by ciliated pores, the earliest condition 
of nephro-gonaducts From this type, by segmentation 
of the trunk or third segment , are derived the Annelida and 
Chordata A 
Later on* these views were given in detail, and the groups of 
Ar chi-chorda and Eu-chorda were shown to differ, among other 
features, in the fact that, in the latter, the third segment or trunk 
became secondarily bilaterally segmented, and that the other archi- 
meric segments became reduced by a migration forwards of the 
newly formed metameric segments. The justification for this view 
lay in the presence of a pre-oral archenteric pouch in Amphioxus 
larva, which early divides into two, and which has been compared 
by many writers to the proboscis-cavity of Balanoglossus , the rest 
of the mesoderm being formed by a series of somites arising from 
the posterior end of the archenteron. 
The main difficulty in the comparison lay in the absence of 
collar-cavities in Amphioxus , or its larva. This difficulty has now 
been removed by the discovery in thife form of a pair of arch- 
enteric pouches which have been identified with the collar-cavities 
of Balanoglossus .f The author remarks — “The whole process of 
mesoderm formation is, therefore, referable to the type formed in 
Balanoglossus , the main difference being that the pouch correspond- 
ing to the trunk coelome of Balanoglossus becomes segmented,” and 
his work is, therefore, a direct confirmation of the views to some 
extent put forward by Morgan, and later, but independently, by 
myself. 
Professor Macbride’s work will undoubtedly give strong support 
to “the theory of the descent of the Vertebrates from a form 
somewhat like Balanoglossus” ; but, from the views expressed else- 
where, I look to Actinotrocha as a proximate morphological pre- 
sentment of the “ form somewhat like Balanoglossus ” rather than 
Tornaria. 
The arehimeric segments persist in the adult as the head-cavity, 
and the pre-oral pit, representing the protoecele, and the first 
myotome and metapleural cavities, which are the mesocoeles (see 
figs. 19 and 20). 
* Quart. Journ. Micro. Science, August 1897. 
t E. W. Macbride, Quart. Journ. Micros. Science , Jan. 1898. 
