148 GEORGE W. TANNREUTHER 
As development progresses the variations between individual 
embryos become less apparent and as far as can be recognized, 
do not affect the final result. 
The history of the cleavage is distinguished by three well 
marked periods, namely: oblique, transitional, and_ bilateral. 
In the first period, which extends to the twenty-four-cell stage, 
the germ layers are differentiated, and the parent cells, which 
give rise to the future organs are definitely marked out. 
The first cleavage is nearly transverse to the median longi- 
tudinal axis of the adult worm. The second cleavage plane 
occurs at an angle of forty-five degrees to the first. The third 
cleavage plane is horizontal and separates the four ectomeres 
above from the four macromeres below. 
Three generations of four ectomeres each are successively 
separated from the macromeres A, B, C and D. The first gen- 
eration of ectomeres (a!, b!, c! and d!'), are formed in a right 
handed direction. The second generation of ectomeres (a2, b?, 
ce? and d?), are formed in a left handed direction. The third 
generation of ectomeres (a’, b’, c? and d*), are formed in a right 
handed direction. From these twelve ectomeres the entire 
ectoderm is formed. 
The ectomere d? gives rise to all or nearly all the ectoderm 
of the trunk, to the nervous system and to the nephridia. 
The oblique type of cleavage is maintained in the division of 
macromeres. At the close of the oblique period the embryo 
consists of twenty-four cells (text fig. 6 and fig. 36). The rela- 
tion of the cleavage cells to the germ layers is as follows: 
A er macromeres..... CNUOGERM ee esi 4 ok ee 
Ile Re Sere mesomere....... MES OGE RITE ores eR re er ens foties lec) coy se 
é IGLOE tte ectoderm 
PAE Se Oe roe me ectomeres. shes 
i 1 (d?)..ectoderm, nervous system, nephridia 
Bilateral division now occurs in some of the ectomeres, while 
others may continue to divide obliquely. The transitional 
period shows both types of cleavage. Oblique cleavage per- 
sists in some of the cells until the fiftieth or more cell stage. 
In the third period, the cleavage becomes essentially bilateral 
