MUSEUM OF COMPARATIVE ZOOLOGY. 177 



for it lies in the same transverse plane in which the otocysts (Plate I. 

 Figs. 3, 4) are situated, and it is found at a region in that plane inter- 

 mediate between the lateral border of the foot and its middle Ime, but 

 considerably nearer the former. The proliferating cells project into the 

 cavity of the foot, and ultimately are separated from the ectoderm. 



Although cells which closely resemble these are found in groups in 

 other parts of the body wall, their nuclei do not become as large as 

 those of the cells destined to form the ganglia. Moreover, the prolifer- 

 ations are constant and most abundant in the regions where the differ- 

 ent ganglia of the nervous system take their origin. Besides, in these 

 cases there is generally a sinking in of the surface of the ectoderm in 

 the same region. 



Somewhat later than at >the stage described, usually on the seventh 

 day, the external conditions still remain nearly the same, the ocular 

 tentacles being perhaps a little more prominent, and the concretions in 

 the shell gland more numerous. 



The cells of the primitive entoderm, which surround the yolk, form a 

 striking feature of the condition at this stage. These entoderm cells 

 are very large, vacuolated, and only slightly stainr.ble. They contain 

 large ovoid nuclei, which are crowded to one margin of the cells by the 

 nutritive contents accumulated in the cells. Each nucleus contains one 

 large deeply stained nucleolus, and a network of chromatic substance 

 (Plate I. Fig. 2). The ectoderm, except over the nutritive sac, consists 

 of elongated cells, whose nuclei are so arranged as to give the appear- 

 ance of two or more layers. The ganglionic cells at this time closely 

 resemble the mesodermic cells, and this makes it difficult to distinguish 

 between the two (Plate I. Fig. 7). 



The internal ends of the primitive nephridial organs are situated one 

 on each side of the head, immediately above and back of the ocular tenta- 

 cles. These organs pass at first forwards and upwards, then in an arch 

 backwards over the nutritive sac, and finally downward and forward. 

 Their external openings are far back in the lateral walls of the body, 

 behind the head region. The organs are readily distinguished in sec- 

 tions by their large slightly stained cells, which are arranged in a single 

 layer around an oval lumen. The large nuclei contain each a single 

 deeply stained nucleolus (Plate I. Figs. 2 and 6). The primitive en- 

 toderm and the nephridial organs retain this histological condition 

 throughout the embryonic stages. 



The cerebral invaginations at first appear as shallow depressions in 



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