450 



INVEETEBRATA 



CHAP. 



in determining the form of the embryo, Zur Strassen imagines that 

 these planes are the seats of some sort of attraction which tends to 

 cause similar planes in other cells to set themselves parallel with 

 them. Further, he imagines that this attraction can wax and wane 

 with the physiological condition of the cell, and by a climax of 

 ingenuity he endeavours to account for the swinging round of the 

 beam of the T, first to the left and then into the middle line, by the 

 successive attraction which two sets of planes at right angles to each 

 other, in the cells A and B, exercise on the planes in the lower cell. 

 The question then arises whether these planes exist in the 



A 



B 



EMST 



FIG. 353. Two diagrams of the 4-cell stage in the development of the egg of Ascaris 

 megulocephala, in order to show the cytoplasmic zones which Zur Strassen assumes to 

 exist in the blastomeres. A, the T condition. B, the rhomb condition. 



One set of planes is indicated by broad bands, another by thin lines. The broad bands in the 

 upper cells first attract the thin lines in the lower cells, and % swings up at right angles to the paper, 

 so that the bands in the upper cells and the lines in the lower are parallel to one another ; then the 

 bauds in the lower cells are attracted by the bands on the upper cells, and set themselves parallel to 

 thesi 1 , and so the condition shown in B is attained. 



unfertilized egg or are formed after fertilization. It occasionally 

 happens that a giant egg is doubly fertilized and gives rise to twins. 

 The axes of these twins are often divergent, and sometimes one is 

 much bigger than the other. Hence we have to assume the exist- 

 ences of two complete sets of the hypothetical planes, and to further 

 assume that each set extends through a different region of the cyto- 

 plasm. The only way out of the difficulty, according to Zur Strassen, 

 is to assume that the planes are manufactured in the cytoplasm ly 

 influences emanating from the zyyote nucleus. 



For this assumption there is corroborative evidence afforded by 

 the development of other forms ; for instance, in the development of 



