EARLY STAGES IN DEVELOPMENT OF VERTEBRATES. 113 



to ascertain what are the general characters of secondary 

 features and how they are produced. Many vertebrates have 

 in the first stages of their development a number of secondary 

 characters which are due to the presence of food material in 

 the ovum ; the present essay is mainly an attempt to indicate 

 how those secondary characters arose and to trace their gradual 

 development. At the same time certain important ancestral 

 characters of the early phases of the development of verte- 

 brates, especially with reference to the formation of the 

 hypoblast and mesoblast, are pointed out and their meaning 

 discussed. 



There are three orders of vertebrates of which no mention 

 has been made, viz., the Mammals, the Osseous fishes, and the 

 Reptiles. The first of these have been passed over because the 

 accounts of their development are not sufficiently satisfactory, 

 though as far as can be gathered from Bischoff's account of the 

 dog and rabbit there would be no difficulty in shewing their 

 relations with other vertebrates. 



We also require further investigations on Osseous fishes, but 

 it seems probable that they develop in nearly the same manner 

 as the Elasmobranchs. 



With reference to Reptiles we have no satisfactory investi- 

 gations. 



Amphioxus is the vertebrate whose mode of development in 

 its earliest stages is simplest, and the modes of development of 

 other vertebrates are to be looked upon as modifications of this 

 due to the presence of food material in their ova. It is not 

 necessary to conclude from this that Amphioxus was the an- 

 cestor of our present vertebrates, but merely that the earliest 

 stages of development of this vertebrate ancestor were similar to 

 those of Amphioxus. 



The ovum of Amphioxus contains very little food material 

 and its segmentation is quite uniform. The result of segmenta- 

 tion is a vesicle whose wall is formed of a single layer of cells. 

 These are all of the same character, and the cavity of the vesicle 

 called the segmentation cavity is of considerable size. A section 

 of the embryo, as we may now call the ovum, is represented in 

 Plate 5, fig. A I. 



B. 8 



