56 SUMMARY OF CURRENT RESEARCHES RELATING TO 



we can judge at present it may be regarded as an organ in which the 

 materials which are of no further use to the body are stored up. 

 Blood-lacunae open into it at its ends and surround it as in regular 

 Echinids, but an efferent duct from it has not yet been detected in any 

 form. 



The mode of origin of the genital products is particularly interesting. 

 The primordial germ-cells lie in a circular genital tube from which arise 

 five saccular outgrowths, into which the germ-ceils wander; these out- 

 growths form the first rudiments of the generative tubes, and the cells 

 not only form the male or female elements, but the general epithelium 

 which, later on, invests the cavities of the generative organs. In the 

 adult these tubes atrophy. 



Dr. Hamann believes that those naturalists take the most correct 

 view of the phylogeny of the Echinodermata, who regard the Asterida 

 as being the most ancient members of the phylum. He discusses in 

 detail the evidence as to the origin of Echinids from Asterids. 



Asterids have five or more radial (ambulacral) longitudinal canals in 

 the ventral walls of the arm, and an oral circular canal ; in regular 

 Echinids these are present, as the neural canals ; in Spatangids the oral 

 ring becomes conuected with the enteric lacunae, as it does also in 

 Crinoids and Holothurians. Asterids have blood-lacunae and an oral 

 blood-lacuna-ring in the septa of the longitudinal canals, but these are 

 wanting in the other groups. Asterids have blood-lacuuae in the septa 

 of the dorsal schizocool spaces at the apical pole, which are present in 

 all Echinoids, placed partly in the arms of Crinoids, and wanting in 

 Holothurians. 



Wandering Primordial Germ-cells in Echinoderms.* — Dr. O. 

 Hamann here deals with a question which he did not fully treat of in 

 his essay on the Histology of Echinoderms (see above). He finds that 

 the primordial germ-cells appear very soon after the larval stages are 

 passed ; they are present in star-fishes and Urchins ■ 5 cm. in diameter. 

 The egg-cell and sperm-cell of all Echinoderms arise from one and the 

 same element of the primordial germ-cell. The canals or genital tubes 

 are placed in Crinoids in the arms, in Ophiurids partly in the dorsal 

 wall and partly in the walls of the bursae, and in Asterids and Echinids 

 in the dorsal walls of the disc. They lie in a septum of connective 

 tissue, in the meshes of which are blood-lacunae ; the sept'un is always 

 found in schizocoelic spaces. The contents of the tubes are, in all cases, 

 cells about 0*008 to 0*01 mm. in size, which exhibit amoeboid move- 

 ments, and have but a small quantity of cell-substance which can be 

 stained. The nucleus is from 0*005 to 0*007 mm. in size, and forms 

 a clear vesicle, in which a well-developed plexus, which ordinarily stains 

 very deeply with carmine, can be made out. In Crinoids the primordial 

 germ-cells come to maturity in the pinnules, which are lateral out- 

 growths of the genital tubes ; in the Ophiurids they pass into the walls 

 of the bursae which are invaginations of the ventral body-wall. In 

 Asterids and Echinids the outgrowths form racemose organs ; the 

 Holothurians probably resemble the Echinids, and in both the adult has 

 no remnants of the tubes. 



The author calls attention to the resemblance between Echinoderms 

 and Hydroid Medusae ; in both there is a migration of primordial germ- 



* Zeitschr. f. Wiss. Zool., xlvi. (1887; pp. 80-98 (1 pi.). 



