306 



INVEKTEBEATA 



CHAP. 



this question in the case of Polygordius, but in the case of the 

 essentially similar development of Patella the question has been 

 answered by Wilson. Herbst has shown that if the segmenting eggs 

 of marine Invertebrates are deprived of their fertilization membranes 

 by shaking, and are then exposed to the influence of an artificial sea- 

 water from which calcium has been excluded, the blastomeres which 

 are formed by successive cleavages fail to cohere, and instead of a 

 Metazoon consisting of many cells, a heap of isolated cells is 

 produced. 



This method was applied by Wilson (1904) to the study of 

 Patella, as follows. The eggs were allowed to segment in normal 

 sea-water until they had attained the 16, 32, or 64-cell stages. They 

 were then transferred to artificial sea- water devoid of calcium, and 



FIG. 242. Two views of the young Haliotis tuberculata to show the formation of the 



adult shell. (After Boutan.) 



A, stage in which the larval shell is still relatively large. B, stage in which the larval shell is 

 relatively very small, and in which the holes in the adult shell have appeared. [B is less magnified 

 than A.] Letters as in two preceding figures. In addition, H, heart; h, holes in adult shell. 



when the blastomeres had separated from one another either 

 completely or so as to form small groups of cells, the isolated 

 blastomeres or groups of blastomeres were returned to normal sea- 

 water and allowed to develop further. The tendency to spontaneous 

 isolation does not, however, immediately cease when this is done, 

 and so, for the earliest stages, Wilson did not use artificial sea-water 

 to produce separation, but divided the first two blastomeres from one 

 another by means of a fine scalpel under a dissecting microscope. 

 Eveu for later stages he found it advisable not to wait till the 

 separation had been completely effected by the artificial sea-water, 

 but as soon as loosening had taken place he completed the separation 

 either by a scalpel or by blowiug a jet of water on the egg through a 

 fine pipette. When the experiment was performed on a larva which 

 had begun its free-swimming life, the method adopted was simply to 

 leave it in the artificial sea-water for twenty-four hours, without 



