582 MESODERM AND CCELOME CHAP. 



been derived from a meroblastic type with abundant yolk like that 

 of the bird, and some Mammals living in Australia at the present day 

 still possess eggs of this type. In the higher Mammalia the yolk has 

 disappeared, as it is no longer needed, the embryo, as we have seen, 

 being nourished by means of a placenta, which will be described pre- 

 sently. The early processes of development are therefore somewhat 

 peculiar, and though the segmentation is holoblastic, the subsequent 

 development is essentially similar to that of the bird, the embryo 

 beginning to appear in a mass of cells (Fig. 156, hy) attached to 



cocl/ 



FIG. 157. Transverse section through the trunk of embryo of frog. 

 ccel. coelome ; coel '. prolongation of coelome into mesodermal segment or 



" protovertebra " ; ent. mesenteron (archenteron) ; msd. mesoderm ; nch. 



notochord ; pr. z>. " protovertebra " ; sg. d. pronephric duct ; som. somatic layer 

 j of mesoderm ; sp. c. spinal cord ; spl. splanchnic layer of mesoderm ; yk. 



yolk-cells. (From Parker and Haswell's Zoology, after Marshall.) 



the upper pole of a large blastodermic vesicle (bv), representing the yolk- 

 sac of a bird but containing a fluid instead of yolk, and being 

 surrounded by a layer of cells known as the trophoblast (ep}. 



In the lancelet alone amongst the triploblastic animals 

 described in this book, does the mesoderm arise as a series 

 of enteroccelic pouches : it is usually at first solid, and may 

 be budded off from the endoderm, from the lip of the 

 blastopore or primitive groove at the junction of the ecto- 



