IV 



COELENTEKATA 



89 



stage anywhere in their ontogeny, but also in the possession of a well- 

 developed nervous and sensory centre at the aboral pole, a part of the 

 body which in other free-swimming Coelentera^ta is the least sensitive 

 portion of the whole surface. The whole development is also of a 

 widely different type from that of other Coelenterata, so that at first 

 sight it is difficult to find any points of resemblance. 



The most primitive type of Ctenophore known is that included in 

 the order Cydippidea. Unfortunately such forms cannot be regularly 

 obtained ; occasionally they turn up in great swarms, and then for 

 years it will be difficult if not impossible to procure them. On the 

 other hand Beroe is a form which is abnormal in many respects, but 

 which can be obtained regularly in the Mediterranean, at least at one 

 season of the year, and it has been made the subject of much experi- 

 mental work. Two species of Beroe occur in the Mediterranean, B. 

 forskalii and B. ovata. The development of these two seems to be 

 identical for all practical purposes and our illustrations will be 

 drawn from each. 



BEROE 



Beroe differs from primitive Ctenophora in possessing an 

 enormously expanded stomodaeum, recalling the cavity of the bell of 

 a medusa, and also in not having any vestige of tentacles. 



We owe our first account of the development of Beroe to Chun 

 (1880), and his account has been supplemented by those of Driesch 

 (1895), Ziegler (1898), and Fischel (1897 and 1898), all of whom 

 approached the subject from the standpoint of Experimental 

 Embryology. 



If Beroe be kept in an aquarium it will deposit its ripe eggs. 

 These are translucent spheres about 1 mm. in diameter, covered by a 

 tough membrane. They contain a large amount of food-yolk. The 

 nucleus is situated near one 

 pole (the upper), and at this 

 pole the most of the proto- 

 plasm of the egg is massed, 

 the rest of the egg consisting 

 of food-yolk. 



Development goes on 

 within the membrane up to 

 the formation of a complete 

 larva, and owing to the 

 transparency of the eggs the 

 greater part of the develop- 

 ment can be studied in the 

 living object ; but larvae 

 can be preserved in osmium 

 acid and embedded in celloidin and then examined by cutting 

 them into series of sections. The egg divides into two, and then 

 four equal segments, by means of furrows which begin at the upper 



FIG. 69. Side view of the segmenting egg of a 

 Ctenophore (Oallianira bialata). (After 

 Metschnikoff.) 



Only one half of the egg is seen ; it is in the 16-cell 

 stage, mac, macromere ; mic, micromere. 



