546 



rable a careful study of many oggs in order to determine the number 

 and relative frequency of the various forms of development; and for this 

 purpose the co-operation of other observers was needful. Dr. J. Me- 

 redith Wilson and Mr. H. L. Osborn, two of my associates in the 

 laboratory, kindly offered their assistance; and the results here recorded 

 were attained through our joint labors. 



The eggs were in most cases kept under constant observation from 

 the time of fertilisation until the occurrence of the first cleavage and 

 were always carefully isolated in small glass vessels and raised to a 

 stage which proved them capable of full and normal development. 



Five well marked modes of segmentation may be distinguished 

 though these are to some extent connected by intermediate forms. They 

 are as follows : 



1) In the most usual case the egg divides at once into 16 spheres 

 which may be of equal size or may show considerable inequality. In 

 succeeding stages the egg divides with more or less irregularity into 32, 

 64, etc. spheres, each stage of activity being followed by a period of 

 rest during which the spheres become closely pressed together and their 

 outlines sometimes become indistinguishable. As a rule, one of the 

 spheres divide into two equal parts at each cleavage but it sometimes 

 happens that one or more of the spheres may pass over one of the periods 

 of activity without dividing. In such cases the division, when it occurs, 

 is into two parts. 



2) In about one third of the eggs studied the first cleavage resulted 

 in the formation of 8 instead of 16 spheres. As in the first case these 

 spheres may be equal or unequal. The subsequent development agrees 

 in all respects with the first case, 



3) The third mode of development was observed in five or six eggs 

 only. At the first cleavage four or five small spheres are formed at 

 one pole of the egg, the remaining portion being undivided. The egg is 

 now closely similiar to a true meroblastic egg like that for instance 

 of Pyrosotna. It then passes usually into a marked period of quiescence, 

 but this is sometimes suppressed. At the second cleavage the remain- 

 der of the egg divides into spheres which are usually somewhat unequal 

 in size. The egg now consists of 16 spheres and cannot be distinguished 

 from those which divide directly into 16 at the outset. 



4) In a single case observed by Dr. Wilson the egg divided di- 

 rectly into 32 spheres which were slightly unequal in size. The sub- 

 sequent development was normal and indistinguishabl(> from the first 

 mode. 



5) In a single case observed by mvself the egg divided at first into 

 two equal parts. Each of these was then imperfectly divided into four 



