136 THE ANCESTRY OF VERTEBRATES 



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 stigmati 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 planulaof 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 Molluscs 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), 



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 c'ose 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 corresponds 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 ectomeies), 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 



