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Chas. B. Wilson 



the ventral surface of tlie tail increase more rapidly than the 

 others. 



4. The blastopore retains its position at the posterior end of 

 the embryo for several days; the yolk cells then begin to withdraw 

 very slowly. At the same time the dorsal lip of the blastopore 

 grows down vertically over the yolk cells, while the archenteric 

 cavity becomes narrowed by the withdrawal of the yolk cells into 

 its posterior ventral portion (fig. 20). This process continues slowly 

 for five or six days, the blastopore growing steadily smaller, until 

 finally, between the 17*^ and 20*^ days the dorsal lip has reached 

 the ventral lip (fig. 1 4). The anus now occupies what was originally 

 the posterior portion of the blastopore and opens ventrally instead 

 of posteriorly. The embryo has become normal in its outward 

 appearance. 



During this growth the anus has changed its position gradually, 

 and has occupied successively every portion of the dorsoventral 

 diameter of the blastopore. 



This gradual closure of the blastopore can be well seen in 

 figs. 17 to 20, which are a series of longitudinal median sections 

 taken from embryos in different stages of development. Figs. 12 

 to 14 show the enlarged posterior ends of another similar series. 



5. We are fully conscious that we are dealing here with ab- 

 normal conditions of development, and that the greatest care must 

 always be exercised in reasoning from the abnormal to the normal. 

 But from the fact that the abnormality in the present instance is 

 chiefly a retardation, from which a normal embryo is ultimately 

 developed, it would seem as if we might infer that in the normal 

 closing of the blastopore the withdrawal of the yolk cells is a more 

 important factor than has been hitherto supposed. 



They are usually regarded as inert and insignificant factors in 

 development, remaining passive while the active pigment cells grow 

 down over them and push them in. From a study of these embryos, 

 however, it would seem that they are really the active agents of 

 their own withdrawal, and when in any way their vitality is im- 

 paired or hindered, the pigment cells have to wait for them to act 

 before they can accomplish their own work. In fact the whole 

 closure of the blastopore, in these embryos at least, seems hinged 

 on the withdrawal of the yolk cells. It is possible that the difference 

 in the modes of closure of the blastopore noted by different authors 

 may depend quite as much upon variations in the activity and mode 



