568 Chas. W. Ilargitt 



In Fig*. 1 0, is shoAvn a sectlon of auother egg- in which likewise 

 the uuclear conditious are veiy similar to those of Fig. 9, but iu 

 the present case evidenees of cleavage are preseut as shown at 

 regioDS marked c.pl. Here there is apparently no dose relation 

 between nuclear activity and that of the cytoplasm. This agaiu is 

 (juite in keeping with conditions well known in other cases. 



In Figs. 12 — 23 are shown a series of simplj outline sketehes 

 illustrating the striking irregularity of cleavage in a large proportion 

 of these eggs. Figs. 12 — 17 (PI. 21) show the first six sections in 

 a series of twelve of a single q^^. The sections begin evidently 

 at the animai pole, if we may assume polarity in these eggs, as 

 only in the first four sections are nuclei present, the later sections 

 showing only the large and very unequal blastomeres of the vegetai 

 pole. In Figs. 18 — 23 (PI. 22) are shown likewise a similar series 

 of another q^^^ but in this case the sections are vertical instead of 

 horizontal, as in the previous case. In this case the six sections 

 are the central ones of a series of fourteen, and show the graduai 

 Progression of cleavage from the animai pole, the other remaining 

 undivided. 



It is probably from observations upon similar cleavage pheno- 

 mena that Ciamician conceived the presence of au epibolic gastru- 

 lation, an Interpretation quite likely to be made where surface views 

 chiefly are under consideration. But as shown in sectional series it 

 is just as evident that Balfour's contention against such a process 

 is indisputable. 



Formation of the Germ Layers. 



Concerning the origin and formation of the germ layers in this 

 and related species views have been hardly less conflictiug than on 

 points already considered. 



Aecording to Ciamician, as previously suggested, an epibolic 

 gastrula is forraed. This seems so obviously improbable, and more- 

 over, so lacking in evidence as to demaud little attempt at refu- 

 tation. It is not difficult to recognize conditions in cleavage which 

 niight easily be construed from superficial aspects as comparable 

 with this form of gastrulation , but many hundreds of sections of 

 eggs in all stages of development have not afiorded the slightest 

 evidence of such a process. 



