40 TEXT-BOOK OF EMBRYOLOGY. 



the dorsal lip of the blastopore extends backward. The lip protrudes, one 

 might say. The extension gradually affects also the lateral lips and finally 

 to a slight degree the ventral lip. This whole process of growth backward, 

 which is due to the rapid cell division in the germ ring most rapid dorsally, 

 less rapid laterally, least rapid ventrally, effects a posterior elongation of 

 the gastrula and a diminution in the size of the blastopore (Fig. 21, E, F, G). 

 This is the first step in the lengthwise growth of the animal as a whole. 

 The whole process of gastrulation has occupied about three hours. 



The account here given differs in one respect from that of the British 

 investigator, MacBride. It has been stated that inflection, or involution, is 

 one of the factors in gastrulation. MacBride maintains that involution 

 does not occur, but that the rapid cell division occurring in the lips of the 

 blastopore produces both ectoderm and entoderm in equal amounts. Cell 

 proliferation is the only process which adds to the number of entodermal as 

 well as of ectodermal components, and this at the same time produces the 

 backward extension of the lips of the blastopore which is recognized as epi- 

 boly. He bases his conclusion on nuclear characters. In the bias tula all 

 the nuclei are vesicular. Soon after gastrulation begins the nuclei of the 

 ectodermal cells become more intensely stainable while those of the 

 entodermal cells retain their vesicular nature, all the invaginated cells pos- 

 sessing the vesicular nuclei. This probably indicates a physiological dif- 

 ferentiation. In the germ ring two types of the rapidly dividing cells can be 

 distinguished, one with vesicular nuclei and the other with deeply staining 

 nuclei. The former are added to the entoderm, the latter to the ectoderm. 

 There is therefore a zone of growth in which cells are produced and added 

 directly to the two layers without inflection round the lip of the blastopore. 



The gastrula is now somewhat elongated antero-posteriorly, somewhat 

 flattened on the dorsal side and is bilaterally symmetrical, with the archen- 

 teron opening to the exterior at the caudal end through the small blastopore 

 (Fig. 2 1 , G) . Even at this time it is not amiss to note a certain fundamental 

 arrangement of structure and anticipate in a measure its biological signifi- 

 cance when carried over into later stages. The ectoderm, the outer layer 

 of the gastrula, is in immediate contact with the environment, which fact 

 implies that response to external stimuli and protection are effected through 

 this layer. In Amphioxus, as well as in certain other lower forms, strong 

 cilia develop on the ectodermal cells by the motion of which the gastrula 

 changes its position. In later stages it will be seen that the nervous sytem, 

 that complex mechanism for transmitting stimuli from one part of the body 

 to another, is developed from ectoderm. The outer layer of the integu- 

 mentary system with certain of its derivatives, primarily protective in 

 nature, is also a product of ectoderm. The archenteron with its lining of 



