MONOGRAPH OF THE EXISTING CRINOIDS. 



433 



comprehensive treatise dealing with the young of Anted on m edit err anea; Wachs- 

 muth and Springer dealt with the development of the crinoids from the jialseon- 

 tological side; 81aden considered the comparative embryology of all the echino- 

 derms; while Vogt and Yung gave the results of some original observations and 

 made some remarks upon the possible interpretation of certain structures. The two 

 Challenyer volumes by P. H. Carpenter, together with his numerous short 

 papers, and the memoirs of Henry Bury on the development of Antedon medi- 

 teiTanea and on the comparative development of the echinoderms call for 

 special mention. Basing their opinion upon the orientation of the interradial 

 angles of the centrodorsal and its significance in the light of their well-known 

 law, Wachsmuth and Springer in 1885 had arrived at the conclusion that Antedon 

 is in reality a dicyclic genus and had suggested that, though no infrabasals had 

 ever been found, these plates might eventually prove to be present in the young. 

 P. H. Carpenter had subjected the law of AVachsmuth and Springer to very spirited 

 criticism and had brought forward what appeared to him to be insuperable 

 objections to it. He was especially emphatic in his denial of the possibility of the 

 occurrence of infrabasals in Antedo7i. It remained for Bury finally to settle the 

 question by his discovery of infrabasals in this genus. 



The only comprehensive work published since the appearance of Bury's paper 

 is that of Dr. Oswald Seeliger on the early stages of Antedon adriatica. In this 

 the segmentation of the ovum and the development of the embryo and larva up to 

 and including the so-called prebrachial stage are treated in minute detail. De- 

 tailed explanations of the differences between his results and those of previous 

 investigators add much to the value of the memoir. 



The development of Antedon adinatica may be summarized as follows: 



CLEAVAQE. 



The first division always begins shortly after 8 o'clock in the morning. It 

 appears as a shallow furrow on the animal pole, which gradually extends to the 

 opposite pole, at first superficial, later sinking deeply into the mass of the egg. 



For a time the two blastomeres appear to be united at the vegetative pole by 

 a narrow bridge of protoplasm, but this eventually gives way, leaving two equally 

 large well circimiscribed cells mutually adherent through a circular area. 



In well-prepared material the nuclei of the two blastomeres can always be 

 made out, lying nearer the animal than the vegetative pole; but the large amount 

 of yoke sometimes makes this more or less difficult. 



In most cases two little bodies can be seen between the cells, which are prob- 

 ably polar bodies. 



Because of the fact that the embryo very soon acquires a monaxial structure, 

 in which the plane of the first cleavage can no longer be determined, it has not been 

 definitely proved whether or not the two first blastomeres correspond to the two 

 body halves of the bilateral larva. The chief axis through the animal and vege- 

 tative poles, however, can be traced through all the stages, and it is not difficult 

 to prove that it corresponds to the major axis of the larva. 



