THE PEVELOPMENT OF THE EARTH-WORM. 211 



Kowalewsky,^ by Ratzel and Warschawski- in Lumbricus 

 agricola, by Rathke^ and Robin in ISephelis vulgaris (in Avhich 

 I have also observed it)j and by Claparede and Metschnikow* 

 in Spio fuVufmosus ; this last observation I am also able to 

 confirm from my own investigations. Such a stage of seg- 

 mentation, though different enough from what is usually 

 found in other animals, certainly cannot be considered ab- 

 normal as in so many other cases, in which irregularities 

 of segmentation are the first and certain sign that the 2^^^^ 

 is under unfavorable conditions and is about to break up 

 before having attained any considerable development; it 

 appears, on the contrary, certain that in these worms this 

 phase, though departing from the general rule, leads on to a 

 healthy development. But the division into three blasto- 

 meres does not occur in all the eggs of L. trapezoides, and is 

 not indispensable to the regular progress of development. 

 Sometimes it happens that the first two blastomeres each 

 produces at the same time a new cell, so that four imme- 

 diately succeed two. The process of segmentation then 

 proceeds in the following peculiar manner: — In the midst 

 of the protoplasm an accumulation of fine granules appears, 

 which is easily distinguished as a dark spot ; this aggrega- 

 tion of the more solid parts of the protoplasm, which, how- 

 ever, has not distinct limits, increases gradually in size and 

 at the same time approaches the surface of the segment. It 

 is advisable to state that this concentration is not to be con- 

 founded with the phenomena which prepare and accompany 

 the first formation of nuclei ; the nucleus appears later in 

 the centre of the mass described above. The mass, as soon 

 as it has arrived at the surface, raises itself above the level 

 of the surrounding protoplasm in the form of a slightly 

 projecting cone ; then, by a narrowing of its base, it sepa- 

 rates from the mother-ceil and a new blastomere is formed. 



This observation agrees in nearly all particulars with that 

 of Kowalewsky on the formation of the small segments in 

 the egg ui EuaxesJ" 



The two blastomeres of the second generation remain very 

 much smaller than the_first ; at first situated symmetrically 

 with regard to the long axis of the egg, they then approach one 

 another, advancing towards the median line, and at the same 



^ " Embryologisclie Studien an Wiirmem und Arthropodeu," *Mem. 

 Acad., St. Petersbour^,' 1871. Tab. vi, fig. 3. 



" ' Zeit. fiir Wiss. Zool.,' T. 18, 186S. 



=» Loc. cit., Tab. i, fig. 6. 



* " Beitrage zur Kentniss der Eutwicklungsgeschiehte der Chsstopoden," 

 ' Zeit. fiir Wiss. Zool.,' 1869, T. 19, Tab. xii, fig. 1, d. 



^ Loc. cit., pp. 13, 14. 



