224 



PRINCIPLES OF EMBRYOLOGY 



causal mechanisms have been produced by the processes of evolution. 

 The importance of the comparative method has been acknowledged in the 

 main chapters dealing v^ith the vertebrates by treatmg iii immediate] uxta- 

 position the two best-known types, the amphibian and bird. In this chap- 

 ter the field of comparison will be widened to include the other vertebrate 

 groups. These will not be treated so fully, since our purpose here is not so 

 much to obtain a detailed and thorough knowledge of the epigenetic 

 processes in each group but rather to provide a sketch of the salient fea- 

 tures for purposes of comparison. 



Each group will first be discussed in turn, and in the last two sections of 

 the chapter an attempt will be made to draw the parallels and point the 

 differences between them. 



I. Cyclostomes and other primitive jish 



The eggs of a representative of the cyclostomes, the river lamprey 

 [Lampetra or Petromyzon) have been studied in some detail in recent years. 

 The egg is about 2 mm. in diameter. Its cleavage is total and very similar 

 to that of the Amphibia. It gives rise to a blastula which is also very similar 

 to the amphibian. At gastrulation a small knob begins to protrude in the 

 spherical embryo and the blastopore appears just below it. The blastopore 

 is small and remains so throughout the whole period of invagination and 

 one sees nothing resembling the yolk plug of the frog. 



Weissenberg (1934, 1936) has studied the gastrulation by means of vital 

 stained marks, and has derived a map of presumptive areas (Fig. ii.i). 

 It shows a considerable resemblance to that of the Amphibia except that 

 the axial mesoderm is, from the beginning, more concentrated towards 

 the dorsal plane. According to Weissenberg, indeed, the presumptive 



Figure ii.i 



Map of presumptive areas in the young gastrula of the lamprey (cyclo- 

 stomes). Close dots, chorda; spaced dots, mesoderm (somitic mesoderm 

 with horizontal lines) ; close vertical lines, neural plate; spaced vertical lines, 

 epidermis; w^hite endoderm. The position of the mesoderm on the ventral 

 side is uncertain. (Modified from Weissenberg). 



