I50 



BIOLOGICAL LECTURES. 



The second method consisted in marking the membrane 

 above the blastoderm and using such marks as points of orien- 

 tation. " The success of the second method depended upon the 

 close adherence between the egg and its membrane. After a 

 very careful test it was found that even after rough treatment 

 the Q^g retained a fixed position relatively to the surface mark- 

 ings. And again by watching individual eggs for some hours 

 it was seen that the Q.g^ did not rotate within the membrane in 

 the early stages or change its orientation with respect to the 

 marks on the egg-membrane. In order to mark the eggs they 

 were removed from the water and partially dried. A needle 

 covered with finely divided carmine was drawn horizontally over 

 the eggs. Small particles of carmine stuck to the membrane 

 in many cases. The eggs were returned to the water and the 

 best marked chosen." 



Jordan and Eycleshymer^ published a joint paper in 1893, 

 from which it becomes plain that in the case of two species 

 of Urodeles and two Aimra there exists no constant relation 

 between the early cleavage planes and the adult axes. " The 

 first and second cleavage planes undergo extensive torsion, and 

 the cells originally on one side of the mid-line come to lie on 

 the opposite side." 



Jordan 2 states, in regard to the cleavage in the newt, that 

 " the total absence of any regularity in the arrangement of the 

 cells is the most conspicuous feature." 



Experiments of a somewhat different nature by Driesch, 

 Wilson, and others have also shown conclusively that the first 

 line of cleavage cannot have the significance which the earlier 

 experiments of Roux would indicate. Recent studies in cell- 

 lineage, together with the work of experimental embryologists, 

 have thrown a great deal of light on the subject here considered, 

 but without furnishing a decisive answer as to the meaning of 

 cleavage. 



The opinion expressed by van Beneden,^ as early as 1883, 

 that all bilateral animals would be found to agree in having the 



^ Journ. of ]\Torph., vol. ix, p. 407. 

 2 Joiirii. of Alorph., vol. viii, p. 269. , 

 8 Archives de Biologie, IV, 1883. 



