FIRST LECTURE. 



THE MOSAIC THEORY OF DEVELOPMENT. 



EDMUND B. WILSON. 



A REMARKABLE awakening of interest and change of opinion 

 has of late taken place among working embryologists in regard 

 to the cleavage of the ovum. So long as the study of embry- 

 ology was dominated by the so-called biogenetic law, so long 

 as the main motive of investigation was the search for phyletic 

 relationships and the construction of systems of classification, 

 the earlier stages of development were little heeded. The 

 two-layered gastrula was for the most part taken as the real 

 starting-point for research, and the segmentation stages were 

 briefly dismissed as having little purport for the more serious 

 problems involved in the investigation of later stages. The 

 cleavage is equal or unequal, total or partial, regular or irregular; 

 the diblastic condition attained by delamination, migration or 

 invagination ; the gastrulation embolic or epibolic : — such 

 were the general conclusions announced regarding the pras- 

 gastrular stages in a large proportion of the embryological 

 papers published down to the time of Balfour and even later. 

 The last decade has, however, witnessed so extraordinary a 

 change of front on this subject that it will not be out of place 

 to review briefly the three leading causes by which it has been 

 brought to pass. 



First, it has become more and more clear that the germ-layer 

 theory is, to a certain extent, inadequate and misleading, and 

 that even the primary layers of the "gastrula" cannot be 

 regarded as strictly homologous throughout the animal kingdom. 

 To assume that they are so involves us in inextricable difficul- 

 ties — such as those for instance encountered in the comparison 

 of the annelid gastrula with that of the chordates, or the com- 



