302 EMBRYOLOGY 



found at this point, where the division is about to take place. In 

 Ctenodrilus pardalis each portion includes only one segment, and thus 

 the budding zones are seen to be repeated segmentally. While they still 

 remain united to one another, the cephalic lobe [prostomium] and 

 the brain, as well as the oral and anal invaginations, are developed on 

 the several portions. The degree of development in which the zones 

 are found increases from behind forwards (Fig. 140 C). 



Some Polychffita and those Oligochseta in which non-sexual reproduc- 

 tion is known are like Ctenodrilus in so far as they also divide in a 

 condition in which no genital organs are present. In the Protula 

 described by Huxley a budding zone arises between the sixteenth and 

 seventeenth segments ; then follows the formation of the prostomium of 

 a new individual in the seventeenth segment. In this case, however, 

 after the separation both individuals become sexually mature. The 

 conditions are similar in the Naidida, in which they were thoroughly 

 studied by Semper. These worms also reproduce by division in the 

 sexually immature condition only. The body of the worm may be first 

 separated by a budding zone into two regions ; then new budding zones 

 are interpolated ; that is to say, fundaments of younger animals arise in 

 the individuals already established. This process is kept up, not, how- 

 ever, serially from in front backwards, but in such a way that individuals 

 of quite different ages come to lie one behind another. When the chain 

 has reached a certain stage of development, it separates into the different 

 individuals, which now reach their final shape by growing considerably, 

 by increasing the number of their segments, and by maturing their sexual 

 organs. 



The cases last considered were, it is true, those of animals without 

 sexual organs, which multiplied non -sexually ; finally, however, all in- 

 dividuals acquire sexual maturity, and are not distinguishable from one 

 another in shape. The conditions are different in those Polychaeta in 

 which new individuals, which become sexually mature, are continually 

 being separated off from the hinder part of the body of an individual that 

 remains [sexually] sterile, a process that is to be placed alongside that of 

 strobilization in the Scyphomedusas. Thus in Autolytus (according to 

 Krohn and Agassiz) there are formed, by the budding of the parent, 

 male and female animals, which lie in a chain one behind the other ; of 

 these the most anterior, the one lying nearest to the parent animal, is 

 the youngest. They separate from the chain according to their ages. 

 The sexually mature animals are essentially different in shape from the 

 budding forms, so that the two were formerly assigned to different 

 species. The sexual animals appear to copulate ; for in a brood-pouch 

 of the female the eggs develop into the worm which subsequently repro- 

 duces by budding. This is therefore a genuine alternation of generations. 



Similar conditions of reproduction are found in some Syllidce, from 

 the budding individuals of which are detached sexual animals, which, by 

 the great development of their parapodia and setae and by their well- 



