50 EMBRYOLOGY 



with a perisarc, and grow out as the hydrorhiza of a new colony, the 

 first hydranth arising from them by a process of budding. In the 

 remarkable Corymorpha, which does not produce colonies, but remains 

 solitary, a very similar abstriction of frustules takes place from the 

 lower part of the polyp. Perhaps we should recognize in this process 

 the last trace of stock-formation. 



Hydroid Polyps with Sessile Gonophores.— The 

 course of the development of embryos which are formed in 

 sporosacs is, according to Allman (No. 15), F. E. Schdlze 

 (No. 46), Hamann (No. 27), and Metschnikoff (No. 12), some- 

 what different from that just described, especially in the 

 formation of the entoderm, and is more closely related to the 

 development of the hypogenetic medusae (see p. 53). It is 

 maintained that in the case of sessile gonophores there arises, 

 by a total and usually equal cleavage, at first a spheroidal 

 solid embryo destitute of a cleavage cavity (a so-called 

 morula stage), the superficial cells of which by more rapid 

 division become separated off as a distinct layer (ectoderm) 

 from the internal cell-mass (entoderm). As is evident, this 

 process is closely related to the formation of entoderm by 

 delamination, which is to be described further on. The 

 bilaminar embryo thus formed elongates and acquires a coat 

 of flagella and, by the dissociation of the entoderm cells, 

 the beginnings of a gastral cavity. In most instances it 

 becomes free as a planula. 



A distinctly unequal cleavage and subsequent formation of a gastrula- 

 stage by epiboly has been described by Ciamician (No. 22) for Tubularia.^ 



^ [The development of Tubularia has recently been thoroughly in- 

 vestigated by A. Bkauer (No. II., Appendix to Literature on Hydroidea). 

 There are two types of cleavage. In the one case it is approximately 

 regular. There are, however, differences in the size of the blastomeres ; 

 but no regular distribution of these is recognizable. At length a coelo- 

 blastula arises. The entoderm is produced by division of the blastoderm 

 cells according to the multipolar type, so that finally the cleavage cavity 

 is filled with entodermal elements given off from the inside of the blastula. 

 This stage looks like a morula, but it is already a bilaminar germ. 

 (Compare also Gerd, No. III., Appendix to Literature on Hydroidea.) 



The second method of cleavage exhibits at first only a multipHcation 

 of the nuclei ; then the cleavage, beginning at the animal pole, progresses 

 toward the opposite side.] 



