March 2, 1906.] 



SCIENCE. 



341 



Extensive cytoplasmic movements during the 

 earlier cleavages are described. 



Conklin differs from Castle in maintaining 

 that the "nervous and muscular systems do not 

 arise from a common primordium and that 

 there is no nerve ring around the blastopore. 



Comparison of the early development of 

 ascidians, Amphiozits and amphibians shows 

 agreement, according to Conklin, as regards 

 axial relations of egg and ■ larva, bilaterality 

 of cleavage, method of blastopore-closure, and 

 probably also as regards origin and position 

 of neural plate chorda and mesoderm. 



But the paper is primarily concerned with 

 egg organization. Hypotheses of formative 

 substances and organization are receiving 

 much attention at present, but have been sub- 

 jected to biit little analysis and criticism. It 

 has seemed desirable, therefore, although the 

 writer does not over-value destructive criti- 

 cism, to attempt in connection with this re- 

 view a brief critical examination of some of 

 the more important conclusions set forth in 

 this paper. 



The earliest indication of polarity consists 

 in the location of the yolk matrix on one side 

 of the nucleus and a slight eccentricity of the 

 latter toward the animal pole. 



Since the yolk matrix is derived, according 

 to the author, from the sphere of the last 

 oogonic division and supposedly contains the 

 centrosome, he is inclined to identify the polar 

 axis of the egg with the cell axis in general 

 and suggests that polarity may thus'be handed 

 down from one generation to another. 



In the living eggs of Cynthia, Ciona and 

 Molgula, when first laid, three regions are 

 distinguishable, a peripheral layer of clear 

 protoplasm in which the test cells lay in earlier 

 stages and which in Cynthia contains sparse 

 yellow granules, the central mass of yolk and 

 the large germinal vesicle. When the nuclear 

 membrane disappears at the beginning of 

 maturation a large amount of clear protoplasm 

 passes into the cell-body and forms a mass 

 eccentric toward the animal pole and distinct 

 from the yolk and peripheral layer. 



The spermatozoon enters on the lower hemi- 

 sphere, apparently at any point within 30° 

 of the vegetal pole. After entrance rotation 



occure and the aster precedes in later move- 

 ments. 



Immediately after the entrance of the sperma- 

 tozoon the yellow and clear protoplasm flow 

 rapidly to the lower pole where the yellow proto- 

 plasm collects around the point of entrance; the 

 clear protoplasm lies at a deeper level. The yel- 

 low protoplasm then spreads out until it covers 

 the surface of the lower hemisphere. 



The withdrawal of protoplasm from the upper 

 pole leaves the maturation spindles closely sur- 

 rounded by yolk. The polar bodies are thus 

 formed at the middle of a yolk-rich hemisphere, 

 which is, however, the animal pole and not the 

 vegetal pole as was claimed by Castle. 



Castle's conclusion that the polar bodies are 

 formed at the vegetal pole of the ascidian egg 

 has stood since its appearance in disagreement 

 with our knowledge of most other eggs. Conk- 

 lin's observation of the movement of the 

 chief protoplasmic portions of the egg toward 

 the vegetal pole is important in that it clears 

 up this error. 



Tlie sperm nucleus moves from the point of en- 

 trance toward the equator in a path which is 

 apparently predetermined. This path lies in the 

 plane of the first cleavage and the point, just be- 

 low the equator at which the sperm nucleus stops 

 in its upward movement, becomes the posterior 

 pole of the embryo. All the axes of the future 

 animal are now clearly established, antero-pos- 

 terior, right-left, dorso-ventral. 



Conklin's statements regarding the path of 

 the spermatozoon appear to the writer to be 

 in serious conflict. If the spermatozoon 

 enters at any meridian of the egg and moves 

 from its point of entrance along a path which 

 corresponds with the plane of the first cleav- 

 age, as Conklin states, the only possible con- 

 clusion would seem to be that the point of 

 entrance determines the plane of the first 

 cleavage. Yet Conklin regards this path as 

 predetermined. The only evidence offered in 

 support of this view is that the spermatozoon 

 apparently does not always take the shortest 

 path to the equatorial region, but sometimes 

 crosses the egg axis on its way. This con- 

 clusion in turn is based on the study of sec- 

 tions. If the copulation path of the sperm is 

 predetermined penetration must be followed 

 by movement into the predetermined meridian 



