ZOOLOGY AND HOTANV, MICKOSCOPY, KTC. 71 



the bottom of the cleavage furrow. In the " cleavage head " radiations 

 are seen in the homogeneous outer layer and ectoplasm, hut they do not 

 extend into the endoplasm. Jieneath the " cleavage head " the endo- 

 plasmic alveoli are considerably compressed. The micromeres, consisting 

 almost entirely of the ectoplasm, cleave very similarly to the whole egg. 

 In the beginning of each cleavage, ectoplasm ic accumulation is seen at 

 the macromere pole. Towards the end of cleavage, an accumulation 

 appears in the micromere region. 



Enucleated fragments destitute of the "cleavage head" do not 

 manifest any division activity. The cleavage plane is not predetermined 

 in the eo;g. An enucleated piece provided with the " cleavage head " 

 divides by itself without the aid of either nucleus or centrosome. Nor 

 is the ray system necessary for the cleavage of enucleated pieces. The 

 cytoplasm above the level of the cleavage head lias little influence upon 

 the accomplishment of the division. The accumulation of ectoplasm 

 over the micromere and macromere poles is formed in enucleated frag- 

 ments in the same way as in the whole egg. 



If the removal of the nucleated portions is effected at the beginning 

 of the division, the cleavage furrow goes on normally, irrespective of 

 the angle of the section. If, however, the same operation is performed 

 upon an egg in which the cleavage has further advanced, the division 

 plane is in most cases turned towards the side of the larger amount of 

 cytoplasm, the enucleated fragments being divided into nearly equal 

 parts. Sometimes, in the egg operated on, the cleavage furrow curls up 

 towards the macromere pole. If the " cleavage head " be split length- 

 wise, a new " head " forms nearly at right angles, and towards the 

 portion with the larger amount of cytoplasm. If an incision is made 

 in the egg in the micromere region, the cleavage is not affected. 



The cleavage furrow tends to divide the egg equally. The ectoplasm 

 flows up and down just as well without the nuclei and centrosomes as 

 with them. This change may be caused by the unequal increase of 

 surface tension due to the internal division phases. The ectoplasm 

 alone is not an active cleaving agent. 



Development of Lucernarians.* — W. Wietrzykowski has studied 

 in particular the development of Halidystiis octoradiatus. The seg- 

 mentation is total and equal, without detinite planes of cleavage. The 

 endoderm is formed by a sort of unipolar migration, which nevertheless 

 approaches embolic gastrulation. An elongated planula, vermiform in 

 fact and without cilia, shows the much vacuolated endoderm cells in a 

 single row. The planul^e fix themselves in groups. 



The fixed larvae give rise typically to four buds, each like a tentacle, 

 and with the structure of a planula. The mouth is formed by simple 

 rupture. In the development of the tentacles, the stages 2, 4, 8, 16, 

 24, 32, 40, and 48 are seen. The perradial and interradial tentacles 

 become marginal bodies, the others form adradial tufts at the summit 

 of the arms. The arms are from the first approximated in interradial 

 pairs. 



At the stage with four tentacles, cellular cords, due to peristomial 



* Arch. Zool. Exp6r„ x. (1912) pp. 1-95 (3 pis. and 50 figs.). 



