2 BIOLOGICAL LECTURES. 



parison of the sexual and asexual modes of development in 

 tunicates, bryozoa, worms and coelenterates. This considera- 

 tion led some morphologists to insist on the need of a more 

 precise investigation of the prae-gastrular stages, and the desi- 

 rability of taking as a starting-point not the two-layered gastrula 

 but the undivided ovum. "The 'gastrula' cannot be taken as 

 a starting-point for the investigation of comparative organo- 

 geny unless we are certain that the two layers are everywhere 

 homologous. Simply to assume this homology is simply to 

 beg the question. The relationsJiip of the inner and outer 

 layers in the vai'ioiis forms of gastriilas must be investigated 

 not only by detei'mining their relationship to the adult body, but 

 also by traeing out the cell-lineage or eytogeny of the individual 

 blastovieres from the begiiining of development y 



The second of the causes referred to was the discovery of the 

 so-called pro-morphological relations of the segmenting ovum. 

 It is now just ten years since Roux and Pfluger independently 

 announced the discovery that the first plane of cleavage in the 

 frog's tgg coincides with the median plane of the adult body 

 (a fact announced many years earlier by Newport, whose obser- 

 vation fell, however, into oblivion). The same result was soon 

 afterwards reached in the case of the cephalopod (Watase) and 

 tunicate (Van Benden and Julin), and for a time it seemed not 

 improbable that a general law had been determined. Later 

 researches disappointed this expectation ; for it was demon- 

 strated that the first cleavage plane may be transverse to the 

 body (annelids, gasteropods, urodeles), or even in some cases 

 show a purely variable and inconstant relation (teleosts). 

 The fact remained, however, that in the greater number of 

 known cases definite relations 6f symmetry can be made out 

 between the early cleavage stages and the adult body ; and 

 this fact invested these stages with a new and captivating 

 interest. 



The third and most important cause lay in the new and 

 startling results attained by the application of experimental 

 methods to embryological study, and especially to the investi- 

 gation of cleavage. The initial impulse in this direction was 

 given in 1883 by the investigations of Pfluger upon the influ- 



