THE DEVELOPMENT OK LKPIDOSIHKN PAIJADOXA. 23 



is reached. The whole thickening of the keel is confiued to 

 the deep layer of the ectoderm — the outer layer passing 

 unaffected over the floor of the groove. As the medullary 

 folds approach one another the groove shallows out and dis- 

 appears. Occasionally, in places, the folds come in contact 

 before the groove has disappeared, so that for a short time 

 they remain separated by a vertical chink (PL 4, fig. 19). As 

 before suggested, this may be looked on as a last trace of a 

 former method of formation of the spinal cord by involution, 

 but any trace of cavity that is so enclosed in Lepido siren 

 is purely temporary and soon disappears. The keel is now 

 (fig. 20) absolutely solid, and there is no indication of the 

 formation of a central canal until about Stage 20 (fig. 21), 

 when the cells of the interior of the neural rudiment are seen 

 to begin to assume a regular arrangement and columnar form 

 on each side of the median plane. 



Along this plane the cells finally split apart, apparently by 

 the secretion of fluid, the cavity in preserved specimens 

 showing an abnndant coagulum. The split appears some- 

 what irregularly, but by Stage 23 it has become continuous, 

 forming a well-marked cavity in the region of the fourth 

 ventricle, and stretching back from this through about three 

 fourths of the extent of the neural rudiment. Anteriorly and 

 posteriorly the neural rudiment still is solid. 



Note upon the Early Development op Protoptekus. 



The egg of Protopterus is much smaller than that of 

 Lepidosiren, measuring only about 3'5 — 4 mm. in diameter 

 (Budgett). Corresponding with this the yolk granules are 

 smaller, averaging about "015 mm. by "01 mm. They have 

 also a characteristic difference in shape, being very frequently 

 lenticular or fusiform. The blastula of Protopterus differs 

 from that of Lepidosiren in the relatively greater depth 

 and volume of the segmentation cavity, and in the greater 

 relative extent of the micromeric region of the egg. The 

 roof of the segmentation cavity is also thicker. 



Gastrulation. — The line of invagination appears nearer 



