316 Recent Literature-. [April, 



velopment of the lizard is chiefly discussed. In the longer chapter 

 on the mammals, several pages are devoted to the early stages in 

 the development of man. 



The remaining two-thirds of the book are, in the present stage 

 of embryological science, of much value to the student, as Profes- 

 sor Balfour here attempts the difficult task of stating the general 

 conclusions derived from a survey of all authentic known facts re- 

 garding the embryology of animals in general. This is done suc- 

 cessfully, the work well deserving the name of a comparative 

 embryology. 



In chapter xi, we are presented with a comparative sketch of the 

 mode of formation of the germinal layers, and the notochord, with 

 a notice of the mode of origin of the allantois and amnion. We 

 notice here a little discrepancy in the author's statement regard- 

 ing the allantoic bladder of the Amphibia, which leaves us some- 

 what in doubt as to the author's final opinion respecting its 

 nature. On p. 108, the author states that the allantoic bladder of 

 the frog "is probably homologous with the allantois of the 

 higher Vertebrates;" on p. 587 he says that it "is homologous 

 with the allantois of the amniotic Vetebrata," on p. 256, it is 

 stated that there is "ample evidence" that the allantois "has 

 taken its origin from a urinary bladder such as is found in Am- 



Chapter xn, observations on the ancestral forms of the Chor- 

 data, is mainly speculative. The author claims that it is clear " 

 from Amphioxus " that the ancestors of the Chordata were seg- 

 mented, and that their mesoblast was divided into myotomes, 

 which extended even into the region in front of the mouth. The 

 mesoblast of the greater part of what is called the head in the 

 Vertebrata proper was therefore segmented like that of the 

 trunk." In the Amphioxus also the only internal skeleton pres- 

 ent is the unsegmented notochord ; a " fact which demonstrates 

 that the skeleton is of comparatively little importance for the so- 

 lution of a large number of fundamental questions." We have 

 for some time inclined to the view that there was a general 

 analogy between the head of an Arthropod and a Vertebrate, 

 more intimate than generally stated, and Balfour's views on this 

 point are of much interest. As to the differentiation of the Ver- 

 tebrate head, he says on p. 260, " In the Chaetopoda, the head is 

 formed of a praeoral lobe, and of the oral segment, while in 

 Arthropods a somewhat variable number of segments are added 

 behind to this primitive head, and form with it what may be 

 called a secondary compound head. It is fairly clear that the 

 section of the trunk, which, in Amphioxus, is perforated by the 

 visceral clefts, has become the head in the Vertebrates proper, so 

 that the latter forms are provided with a secondary head like that 

 of Arthropods." Hence Balfour considers that the part of the 

 head containing the fore-brain is probably "the equivalent of the 



