REVIEW OF SPENGEL’S MONOGRAPH ON BALANOGLOSSUS. 409 
intermediate between those observed in Amphioxus and the 
Elasmobranch embryo. Ch!. denotes the primary cuticular 
sheath of the notochord, Ch®. the mesoblast concerned in the 
formation of chondroid tissue, ‘membrana reuniens,’”’ and 
cartilage, in Balanoglossus, Amphioxus, and Scyllium respec- 
tively. The actual skeletal substance is indicated by dotting. 
We see that whereas in Amphioxus the sclerotome is a single 
hollow outgrowth on each side, in Balanoglossus it is repre- 
sented by a number of more or less solid outgrowths, and in 
Scyllium by a single solid one. 
Spengel’s topographical arguments are (1) that the so- 
called notochord appears late in development ; (2) that it shows 
no relation to the blastopore or its line of closure, such as has 
been described in Elasmobranchs; and (8) that it is on the 
ventral side of the dorsal vessel. The circumstance that the 
notochord and other chordate features appear comparatively 
late in the development of the Enteropneusta is one of the 
most interesting features of the group. They are, as Professor 
Lankester has pointed out, the only members of the chordate 
phylum which take us into ‘ prechordate”’ times, and give us 
some reliable indication of the direction in which to look for 
the ancestry of the Chordata. The reason for the precocious 
appearance of notochord and nerve-cord in Vertebrate develop- 
ment is the hurrying over of the earliest stages whilst the 
embryo is still within the egg-membrane. In the larva of 
Balanoglossus Kowalevskii the notochord, nerve-cord, 
and the first pair of gill-slits are present at or immediately 
after the time of hatching. A precisely similar instance of the 
precocious appearance of an organ in embryonic as opposed 
to larval development is afforded by the Vertebrate oviduct. 
This appears in the chick on the fifth day, but in the frog only 
after terrestrial life has commenced and the tail is being 
absorbed. 
The appearance, in the development_of Elasmobranch and 
other heavily yolked eggs, of the formation of the notochord 
and nerve-cord by the coalescence of the two lips of a slit, 
giving the impression that in ancestral forms hoth were paired 
