Vol. XXXVII. September, IQIQ. No. 3. 



BIOLOGICAL BULLETIN 



CLEAVAGE AND MESENCHYME FORMATION IN 

 TOXOPNEUSTES YARIEGATUS. 



MARY J. CxUTHRIE AND HOPE HIBBARD, 

 BRYN MA\VR COLLEGE. 



This investigation was undertaken for the purpose of deter- 

 mining the origin of the primary mesenchyme in Echinoidea. 

 Although numerous researches have been carried out on the early 

 stages in the development of echinoids, the question of the origin 

 of the mesenchyme has never been definitely settled. 



MacBride, 1 writing of Echinus csculentns, states that "this 

 mesenchyme originates, almost certainly, from the descendants 

 of the smaller micromeres." The figures which Boveri (1901) 

 gives of the eggs of Strongylocentrotus limdus indicate that this 

 is true, and it is believed that unmistakable evidence is herein 

 presented that in Toxopneustes variegatus (Lytechinus variegatus) 

 the mesenchyme arises from the micromeres. 



MATERIAL AND METHODS. 



The material on which this investigation was carried out was 

 obtained by Dr. D. H. Tennent, during the summer of 1908, in 

 the course of work at the United States Fisheries Biological 

 Station, Beaufort, N. C. 



The segmenting eggs were fixed in either picro-acetic or sub- 

 limate-acetic. In both cases the solutions were two per cent, 

 acetic acid. The observations recorded here were made on 

 whole embryos stained with borax carmine and s'afranin, and on 

 sectioned material stained with borax carmine or iron haematoxy- 

 lin. 



In addition to the above material, Dr. Tennent lent camera 



1 " Text-book of Embryology," p. 507. 



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