4 CONK LIN. . [Vol. XIII. 



time a number of very valuable papers have appeared on this 

 subject of "cell lineage," as Wilson ('92) has aptly termed it. 

 The results of such work are no longer as novel as they were 

 four or five years ago, and yet the general interest in the 

 subject has greatly increased, and that, too, in spite of the fact 

 that there is a growing school of biologists who believe that 

 individual blastomeres have no necessary relation to future 

 organs. The subject of germ layers is no longer so important 

 as it was once considered ; in fact, the theory of the homology 

 of the germinal layers has met with so many difficulties of late 

 that it is now generally maintained only in a greatly modified 

 form. However, the fundamental idea which was prominent in 

 germ-layer discussions is of vital interest to-day. In the whole 

 history of the germ-layer theories I see an attempt to trace 

 homologies back to their earliest beginnings. This problem 

 is as important to-day as it ever was, and whether one find 

 these earliest homologies in layers or regions or blastomeres 

 or the unsegmented ovum itself, the quest is essentially the 

 same. 



Within this question of the earliest homologies is included 

 another of great present interest, viz., the significance of 

 cleavage. Is it an orderly sifting of materials, a " mosaic 

 work," or, as Driesch ('93) has maintained in the case of the 

 echinids, a mere quantitative division of homogeneous material .'' 

 Can the cells of cleaving eggs be compared with each other as 

 the organs of adult animals can .■' Can one properly speak of 

 the homology of blastomeres .-' Are the chief axes and regions 

 of the egg or embryo homologous in different animals .-* And 

 finally, are the causes of the various forms of cleavage to be 

 found primarily in the constitution of the egg itself, in other 

 words, in the internal conditions, or rather in the external condi- 

 tions, such as pressure, surface tension, gravity, etc. .'' I know 

 that in these days, when "all the world shakes eggs," it may 

 be hazardous to risk an opinion on these questions which is not 

 based on experimental work. And yet, while fully recognizing 

 the value of experimental embryology, we ought not to forget 

 that " Nature is continually performing some very remarkable 

 experiments in her own way," and I believe we need to know 



