CHAPTER XXVII 



THE DEVELOPMENT OF THE FROG 



The ova of the frog are extruded by the female in the early 

 spring, and are fertilised upon extrusion by the spermatozoa 

 of the male. Each ovum is about the size of a No. 3 shot, 

 and is surrounded by a transparent gelatinous coat secreted 

 by the walls of the oviduct. These coats swell up on 

 contact with water, and adhere to one another, forming 

 gelatinous masses in which the ova are imbedded, the 

 masses being familiarly known as frog's spawn. Quantities 

 of it may be obtained in ditches and ponds in the month 

 of March ; the ova can easily be kept in aquaria, and are 

 particularly favourable objects for the study of segmentation. 

 The later stages are not quite so easily studied, because the 

 abundance of black pigment and consequent opacity of the 

 embryos does not admit of their being studied as transparent 

 objects, and the relatively large quantity of food-yolk makes 

 it difficult to cut good sections of the earlier stages. With 

 a little care, however, satisfactory sections can be made, and 

 all the stages of development may be followed out with great 

 thoroughness. 



The segmentation and formation of the blastula have already 

 been described (vol. i. p. 122). The subsequent events may 

 be briefly sketched as follows : — The blastula, by a modified 

 process of invagination, becomes a gastrula with an outer layer 

 or epiblast and an internal cavity- or archenteron lined by 

 hypoblast. During the formation of the gastrula a portion 

 of the inner cells is separated off as a sheet of mesoblast lying 

 between the other two layers. The epiblast of the dorsal 

 surface is raised to form a pair of longitudinal folds enclosing 

 a groove, and the folds meet and unite in the middle line, 

 converting the groove into a tube, the cavity of which becomes 

 the canal of the spinal cord and the ventricles of the brain, the 

 walls of the folds being converted into the cerebro-spinal nervous 

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