70 
direction in which it moves forward. Already during the 
egg cleavage, which exhibits certain points of resemblance 
with the spiral type of Polyclads, Nemertines, Annelids 
and Molluscs (DELSMAN, 1918), the polarity becomes evident. 
The opposition of an animal pole at the anterior side of 
the swimming colony and a vegetative pole at the opposite 
end of the main axis is also pronounced in the different 
structure of the cells of both sides, the red stigmata charac- 
teristic for light-sensitive Flagellates being best developed 
in those of the animal half, less developed near the equator 
and absent in the neighbourhood of the vegetative pole. 
Here, on the contrary, the cells communicate with each 
other by a greater number of plasmatic connections, plas- 
modesms, serving for the transport of food. 
In the blastula and already in the egg we find this same 
contrast, expressed by the names animal and vegetative. 
In cases where a free-swimming blastula is found it moves 
as well with the animal pole forwards and rotating round the 
main axis. The same holds good for the planula of Coelen- 
terates and other pelagic larvae, inclusive that of Amphioxus 
(HATSCHEK, 1882, p. 37). The region round the animal pole 
genera!ly develops into a sensory and, as a consequence, 
a nervous centre. One of the oldest sense-organs is the 
tuft of long cilia at the animal pole, known as the apical 
organ, which we find in the larvae of Polyclads, Nemertines, 
Annelids, Bryozoa and Mulluscs and, in a very specialized 
form, in Ctenophores. Though it is less distinct here, such 
an organ is to be found also in Echinoderm- and Bala- 
noglossus-larvae (Deuterostomia). 5 
Of other sense-organs in the neighbourhood of the animal 
pole must be mentioned in the first place that of the optic 
sense which already in Volvox begins to concentrate here. 
It may be observed that also in the Deuterostomian Bala- 
noglossus two pigment-spots are found close to the animal 
pole in the larva. Finally, the olfactory grooves and the tentacles 
in Protostomia are found on the prostomium or praeoral lobe. 
This region round the animal pole, the apical plate of the larva, 
evidently corrêsponds to the anterior half of the blastula and of 
Volvox, as in animals with the spiral cleavage type it originates 
from the four superior cells of the eight-celled stage (first 
quartet of ectomeres), i. e. from the animal half of the blastula. 
The ultimate fate of the praeoral lobe in the animal king- 
dom may be very different. In most groups it develops into 
