SEVENTH LECTURE. 



CELL-DIVISION AND DEVELOPMENT. 



J. PLAYFAIR McMURRICH. 



Within the last few years the science of embryology has 

 undergone a remarkable development, especially along two 

 lines. In earlier years the dominant idea was a phylogenetic 

 one, embryologists seeking to discover from the individual 

 development facts which might contribute to the formulation 

 of a correct phylogeny for the species or group under consid- 

 eration. They were content accordingly to carry the ontogeny 

 back to the formation of the primary germ-layers, that is, back 

 to a stage supposed to represent an ancestral diploblastic an- 

 cestor, the Gastraea ; for still earlier stages a brief statement 

 as to whether the gastrulation was embolic or epibolic or that 

 the segmentation was total, regular or irregular, centrolecithal 

 or meroblastic, being considered sufficient for the most part. 

 Not but that there were striking exceptions to this prevailing 

 indifference regarding the details of cleavage, and these have 

 borne fruit in the awakening of embryologists to the impor- 

 tance of tracing out the cell-lineage of organs and of gaining 

 thereby a deeper insight into the phenomena of differentiation, 

 the line of research which is so characteristic of recent embry- 

 ology, and which forms one of the paths along which the sci- 

 ence has progressed. Concomitant with this new departure, 

 which may be termed the cytogenetic method of embryology, 

 in contrast to the phylogenetic, came the development of the 

 science of experimental or physiological embryology, from 

 which so many important deductions concerning the funda- 

 mental constitution of the ovum have resulted. 



Both cytogenetic and experimental embryology have had to 



