534 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Echinoderma. 



Larval Development of Echinus microtuberculatus.* — H. Schmidt 

 has studied one hundred and five stages, representing a continuous 

 series, from the developed blastula before the formation of the primary 

 mesenchyme up to the pluteus with arms and fully-formed left vaso- 

 peritoneal vesicle. Abnormalities were exceedingly rare, and almost 

 never occurred in the later stages. Only isolated blastulae were observed 

 with pathological mesenchyme and dwarfed plutei. The development 

 was remarkably even in its course, and at the temperature (17° C.) rapid. 

 At 36 hours the gut had reached the opposite end, and in 41 hours 

 plutei with well-developed arms were formed. From 17 to 22 hours' 

 larvae showed, besides the primary mesenchyme, an animal pole consist- 

 ing of a thick plate of blastoderm cells. At the end of the blastula 

 stage there arises at the vegetative pole a specially differentiated 

 plate, provisionally designated " endoderm plate." Very soon this plate 

 is invaginated (20th hour). Two hours later there is present a well- 

 developed gut. During the following hours (22 to 26) the formation 

 and migration of the secondary mesenchyme is completed. At the 

 same time the gut is extending towards the opposite side, while at the 

 end of the gastrula period alterations in form and size are evident 

 which mark the transition to the pluteus. The first hint of the mouth 

 appears to be about the 30th hour, while the swelling of the blind end 

 of the primitive gut to form the later vaso-peritoneal vesicle appears at 

 33 hours. The ciliated band inaugurating the pluteus stage is seen an 

 hour later, and by 36 hours this stage is clearly defined. The vaso- 

 peritoneal sacs now constrict themselves off. At the 38th hour the 

 gut is in three parts, and it is only after 41 hours that the mouth is 

 formed. The rest of the time up to 45 hours is occupied by the de- 

 velopment of the larva to the typical elongated pluteus form, and the 

 growth of the arms. 



" Pentasomaea " Theory .f — E. Herouard states what he calls the 

 " Pentasomcm' 1 '' theory of the Echinoderm body, according to which 

 somites arise in groups of fives, each of the groups forming a " penta- 

 somic vesicle." The five somites potentially included in a pentasomic 

 vesicle do not begin to be individualised until the vesicle has been 

 liberated into the blastoccel cavity. The five somites of a pentasomic 

 vesicle in Echinoderms are only partially individualised ; their cavities 

 remain in communication through the residue of the pentasomic vesicle 

 which formed them — a residue that represents the enterotomes of the 

 somites. He compares the pentasomic vesicle of Echinoderms with the 

 enteroccelic vesicle in Amphioxus, which has four specialised regions : 

 myotome, sclerotome, enterotome, and gonotome. The five somites of 

 the superior pentasomic vesicles in Echinoderms form the radial canals, 

 those of the lower vesicles form the general radial cavities. The general 

 result of a theory, which is somewhat difficult to follow, is to the effect 

 that Vertebrates and Echinoderms have had a common ancestor. A 



* Verh. Phys. Med. Ges. Wiirzburg, xxxvi. (1904) pp. 297-336 (5 pis), 

 t Bull. Soc. Zool. France, xxix. (1904) pp. 70-81 (9 figs.). 



