258 GENERAL EMBRYOLOGY 



forms each of the four or eight cells has already become differ- 

 entiated. In forms like the Ascidians (Conklin) true differ- 

 entiation has commenced in the uncleaved ova and has been 

 carried to a very pronounced degree. And it is not at all 

 unlikely that these differentiations of the undivided egg may 

 represent the most essential and most fundamental differen- 

 tiations of the organsim. This aspect of cleavage cannot be 

 discussed satisfactorily here without encroaching widely upon 

 the subject of the next chapter and it is therefore left at this 

 point. 



It is to be noted, however, in conclusion that while cleavage 

 may have an important chemical significance of general char- 

 acter, and a general physiological significance, yet the process 

 is primarily a specific process of development and not mere 

 cell multiplication. The process is closely related in all its 

 details to the structure of the egg and also to the structure of 

 the adult, since this too is similarly related to egg structure. 

 Many of the details of cleavage do not occur according to phys- 

 ical laws based upon space and time relations of the parts, 

 but departures from what we should expect on the basis of 

 such laws may result, (a) from the historical factor of the 

 relationships of organisms and the process of descent, (6) from 

 the teleological factor, for cleavage has a prospective signifi- 

 cance, looking forward, as well as a retrospective significance 

 cleavage has its promorphology as well as its morphology. 



REFERENCES TO LITERATURE 



BALFOUR, F. M., (See ref. Ch. III'.) 



BEARD, J., (See ref. Ch. III.) 



BOVERI, T., Die Polaritat von Ovocyte, Ei und Larve des Strongylocen- 

 trotus lividus. Zool. Jahrb. 14. 1901. 



BRESSLAU, E., Beitrage zur Entwicklungsgeschichte der Turbellarien. 

 I. Die Entwicklung der Rhabdocolen und Alloicolen. Zeit. wiss. 

 Zool. 76. 1904. 



CONKLIN, E. G., (See ref. Ch. II, III.) 



DRIESCH, H., Entwicklungsmechanische Studien. VIII. Ueber Varia- 

 tion der Mikromerenbildung. (Wirkung von Verdiinnung des 

 Meerwasser). Mitt. Stat. Neapel. 11. 1893. 



