56 M. E. Bessels on the Siyecies ofAtax 



recently A. Brandt has studied the developmental history of 

 the Libellulidffi and Hemiptera with special reference to the 

 embryonal envelopes. 



The Acarida?, however, had not been taken up by any one 

 in the manner required by the present state of science. For a 

 considerable time I had taken pity upon these neglected crea- 

 tures, and investigated the development of Atax, Pliytoiyus^ 

 Tetranychus telarius, Sarcoptes^ and some other forms. When 

 I was on the point of publishing my results (I only waited for 

 the beginning of May in order to till up some deficiencies in 

 the development oi Phytopus)^ I was not a little sm-prised at 

 finding in the last part of Siebold and Kolliker's ' Zeitschrift' 

 a memoir by Claparede*, elaborated in his usual masterly 

 manner, which rendered the publication of the developmental 

 history of those species which I had investigated in common 

 with Clapar^de almost superfluous, inasmuch as our results 

 essentially agreed. 



The development of Atax ypsilophorus, some points in 

 wliicli will be here indicat^l, was described in its broad fea- 

 tures by P. J. van Beneden as early as the year 1848t. But 

 precisely the most remai-kable circumstances escaped that 

 observer, otherwise so accurate ; and this may be due to the 

 fact that he probably made use of a different eg^ for the in- 

 vestigation of each stage of development. In a letter which 

 I sent to Van Beneden at the beginning of September 1868, I 

 mentioned, en passant^ that the results which I had obtained 

 with regard to the development of A tax could not be brouglit 

 into accordance with his. In connexion with a memoir upon 

 the spherical organ in the Amphipoda % (sent to press in No- 

 vember 1868), I mentioned the occurrence "of an embryonal 

 envelope of extremely peculiar characters in the species of 

 Atax from Unio and Anodonta,^^ and also the amoeboid cells 

 found between this envelope and the embryo, which are called 

 haemamoebas by Claparede. 



As has already been stated, my results agree with Clapa- 

 r^de's in all the principal points. In the observation of the 

 formation of the blastoderm, however, I have been rather 

 more fortunate than the above-named natm-alist, who was un- 

 able to observe that process. How long after the deposition 

 of the eggs the blastoderm makes its appearance, no one can 



* Studien an Acarideii, pp. 44/5-54G. 



t " Eecherches sur I'llistoire natarelle et le Developpement de VAtax 

 ypsUoph(/)-a,'^ Memoires de I'Acad. Roy. de Belgique, tome xxiv. 



\ " Einijre Woiie liber die Entwickehingsgeschicbte und den morplio- 

 logischen Werth der kiigelfcirmigen Organe der Amphipoden," Jeuaisdie 

 Zeitschrift fiir Medicin und Naturwissenchaften, Bd. v- Hft. 1. p. 98. 



