256 DKVKLOPMENT OF A XOX-AQUATIC TADPOLE. 



that- nuclei (n) have crept into the superficial protoplasmic layer 

 from the syncytium in advance of the growing blastoderm. 

 Nuclei (n.y.) appear to escape from the superficial protoplasmic 

 layer and invade the neighbouring yolk-mass (Fig. ^b). 



It is clear that the early stages are much modified by the 

 presence of the excessive quantity of yolk. As far as the material 

 goes it would appear that segmentation is confined to the forma- 

 tion of a few large, incompletely cut off cell-masses or blasto- 

 meres lying above the yolk-mass and communicating with it by 

 the peripheral layer of protoplasm. Subsequently the nuclei 

 divide without the separation of cells and the partially separated 

 masses fuse and form a thick sjmcytium with numerous nuclei, 

 and some of these creejD into the peripheral protoplasm. Prob- 

 ably the superficial portion of the syncytium separates off as an 

 ectoderm while the remainder becomes mesoderm. Thus, a 

 massive blastoderm, never completely suiTounds the yolk as in the 

 development of the common frog, but the general condition is very 

 similar to that seen in the developing egg of the bird. With 

 the available material it cannot be said whether a definite 

 primitive streak is formed. The origin of the archenteron is 

 also not exliibited. but in the earliest stages in which the gut 

 is present the endodei'm is so densely crowded M-ith yolk and 

 is so intimately associated with the yolk-mass that it is unlikely 

 that it ai'ises directly from the blastoderm, but it is doubtless 

 differentiated from the inid-dorsal portion of the yolk-mass (PI. 

 Ill, fig. 2a). 



The next stage which I possess shows the embryo with a 

 well-developed neural tube, notochord and sub-notochordal rod. 

 but the archenteron is still in a remarkably undifferentiated 

 condition, with only a roof indistinctly marked off from the 

 surrounding yolk The mesoderm has crept round from the 

 region of the blastoderm and encloses the yolk with a very 

 thin and scarcely continuous layer on the inside of the thin 

 ectoderm layer. Th'e ectoderm layer shows some signs of being 

 divisible into an inner and outer layer. 



Slightly later the whole yolk-mass splits up into large oval 

 cells (PI. Ill, fig. 2b), each bearing a large bladder-like nucleus. 

 These nuclei appear to be derived from nuclei which have 

 wandered into the yolk-mass from the thin, syncj^tial, superficial 

 layer. 



This cellular condition of the yolk lasts only a brief period, 

 and very soon the majority, if not all, of the nuclei break down ; 

 the cells run together and the yolk-globules become almost free, 

 for by this time the reticulum of protoplasm has become greatly 

 reduced. 



The Gut. 



The arclienteron becomes more especially differentiated at the 

 anterior and posterior ends,' and takes a straight course through- 

 out the length of the embryo. In the middle region it is only 

 indistinctly separable from the yolk. Anteriorly it expands 

 considerably to form the pharynx region, while posteriorly it also 



