CNIDARIA 51 



But his investigations have been refuted by Hamann (No. 27), Metschni- 

 KOFF (No. 42), and Conn (No. 21), according to whom the development of 

 the egg of Tubularia takes place in accordance with the type described 

 above. On the other hand, it appears as though Tichomiroff (No. 47) 

 had expressed himself in favour of Ciamician's observations. The bi- 

 laminar embryo has at first the form of a cake, but soon becomes spindle- 

 shaped, owing to the budding forth of two tentacles at opposite points. 

 Then follows the formation of the gastral cavity and of new tentacles in 

 the equatorial plane. The latter are at first curved toward the aboral 

 side. The embryo now generally undergoes an elongation in the direction 

 of the chief axis ; and while at its oral pole the beginnings of the oral 

 tentacles appear and the mouth -opening breaks through, and while the 

 main tentacles curve orally, the posterior end becomes narrower and to a 

 certain extent constricted off from the main body by a circular furrow. 

 With this the so-called aciinula- stage (Fig. 19) is reached, and the small 

 polyp quits the mother (gonophore) for the purpose of attaching itself and 

 growing up into a new colony. The agreement with the development of 

 the larva of the ^ginidse to be described further on (p. 57) is note- 

 worthy. 



The egg of Hydra ^ develops in an ovarium which belongs to the 

 ectodermic layer of the body-wall of the polyp, and which has arisen by 

 an increase of the cells of the so-called interstitial tissue. Of the cells 

 composing the ovarium only one (in rare cases two) is developed into a 

 mature egg, whereas the remaining ones serve as food for it, and are in- 

 corporated into the egg by jneans of its pseudopodia. The mature egg, 

 which contains numerous yolk elements called pseudo- cells, escapes by 

 the rupture of the enclosing ectodermic layer of the parent, to which it, 

 however, remains attached for a long time. The part of the egg which 

 is directed away from the body of the mother marks the animal pole, 

 that which adheres to it the vegetative pole, of the egg. Then follow the 

 detachment of the polar globules and fertilization. The development of 

 the egg has been studied by Kleinenbkrg (No. 32), Korotneff (No. 35), 

 andKERscHNER (No. 33). According to Kerschner, a solid morula is not 

 formed ; but there is produced by total and tolerably equal cleavage a 

 blastula, from the lower (vegetative) pole of which there is a migration 

 into the cleavage cavity of cells which go to form the entoderm. In this 

 case, then, the entoderm arises by polar ingression ; and since in Halecium 



1 [In regard to the development of Hydra the reader is referred to the 

 important recent investigations of A. Brauer (No. I., Appendix to Litera- 

 ture on Hydroidea). A coeloblastula with a large central cavity is pro- 

 duced by total and equal cleavage. The formation of the entoderm is 

 multipolar, and results from the inward migration or the division of the 

 blastoderm cells. The ectoderm secretes an outer and an inner germinal 

 membrane ; it is itself preserved, however, and persists as the permanent 

 ectoderm. The layer of interstitial cells arises from the ectoderm. The 

 future oral pole is identical with that of the polar bodies.] 



