No. I.] THE EMBRYOLOGY OF CREPIDULA. 33 



Neritina. Nor can it in all cases be explained by assuming^ 

 that in each o.^^ capsule there is a struggle for existence, and 

 that the fittest survive while those less hardy are destroyed, 

 since in some forms, e.g., Neritina, the development does not 

 proceed far enough to introduce such a struggle. From the 

 very beginning of development the ova are divided into two 

 classes, those which segment regularly and develop into normal 

 embryos, and those which divide irregularly and never form 

 embryos at all. Blochmann thinks that in Neritina the eggs 

 which do not develop have not been fertilized, while McMurrich 

 believes that too little yolk was furnished for the number of 

 eggs produced, and that, therefore, some of the eggs broke 

 down and were used as food by the embryos which survived. 

 " This process," he says, " might have been seized upon by 

 natural selection, and increased by it until it became a regular 

 process of development." 



I am inclined to believe that in different species different 

 causes may have been operative in producing these abnormal 

 forms. In Neritina, Purpurea and all other forms in which 

 the development of some of the ova goes no farther than a few 

 irregular cleavages, the most probable cause of such non- 

 development seems to be the lack of fertilization, for if McMur- 

 rich's supposition is the correct one we should expect to find 

 the ova which undergo development larger than those which 

 do not, but there is no evidence of such disparity in size. On 

 the other hand, in those forms in which the abnormalities do 

 not appear at an early stage and with great regularity, e.g., 

 Crepidula or Urosalpinx, in which they may or may not be 

 present, and if present may occur at any stage, in such cases I 

 a,m convinced that the abnormal forms are the result of un- 

 favorable environment, e.g., lack of oxygen, presence of bac- 

 teria, mechanical pressure, etc. 



c. history of the cleavage. 



Nomenclature. 



The question of an accurate and convenient nomenclature 



for the various cells of the cleaving ovum, while of no scientific 



value, is, nevertheless, of considerable practical importance.. 



