GASTRULATTON OF VERTEBRATES. 



Fig. 24. FOUR STAGES IN THE DEVELOPMENT OP AMPHIOXUS ILLUSTRATING THE FORMATION OF THE 



GASTRULA. (Hatschek.) 



I. Spherical blastoderm ; the cells at the lower pole are larger than the others, and filled with 

 granules. 



II. Invagination of the lower pole producing a cupping of the vesicle. 



III. Completion of the imagination ; the blastoderm is now bilaminar, and forms a cup with 

 narrowed mouth, the blastopore, bl, and a double wall of epiblast, ep, and hypoblast, hy (or primitive 

 ectoderm and primitive entoderm). 



IV. The ovum is now elongated ; the cavity of the gastrula forms a primitive alimentary canal, the 

 orifice of which is the blastopore, which is directed dorsally. Extending from this along the dorsal 

 surface (right in the figure) a shallow groove is seen in optical section : this is the rudiment of the 

 nervous system. 



ologists that cells from the lateral parts of both primary layers are added to the 

 intermediate layer, and assist in its extension. According to the observations of 

 Bonnet in the sheep, there is an addition to the middle layer from the peripheral 

 (thickened) portion of the hypoblast ; this has been long held to be the case with 

 the blastoderm of the bird, and the cells thus derived (parablastic) have been 

 considered to have the special function of forming the connective tissues and blood. 

 Whether, however, this is actually so, must be regarded as at present undecided. 

 However produced, the appearance of a middle layer causes the originally bilaminar 

 blastoderm to be trilaminar, and its three layers have received the names of ectoderm, 

 mesoderm, and entoderm, or epiblast, mesoblast, and hypoblast. 



The g-astrula condition of the vertebrate ovum. It will be observed that in the 

 mammal the two primary layers of the blastoderm, at least their principal part, are formed by 

 a separation into two strata of the cells of the inner granular mass which occupies the 

 interior of the ovum after segmentation. The bilaminar condition may therefore be said to 

 result from a process of delamination in an originally simple mass or stratum. But in 



