PROMORPIIOLOGICAL RELATIONS OF CLEAVAGE 2/9 



the embryo is definitely related to it. The great embryologist 

 pointed out, further, that the early cleavage-planes also are defi- 

 nitely related to it, the first two passing through it in two meridians 

 intersecting each other at a right angle, while the third is transverse 

 to it, and is hence equatorial. 1 Remak afterwards recognized the 

 fact >2 that the larger cells of the lower hemisphere represent, broadly 

 speaking, the " vegetative layer" of von Baer, i.e. the inner germ- 

 layer or entoblast, from which the alimentary organs arise ; while 

 the smaller cells of the upper hemisphere represent the " animal 

 layer," outer germ-layer or ectoblast from which arise the epidermis, 

 the nervous system, and the sense-organs. This fact, afterwards 

 confirmed in a very large number of animals, led to the designation 

 of the two poles as animal and vegetative, formative and nutritive, or 

 protoplasmic and deutoplasniic, the latter terms referring to the fact 

 that the nutritive deutoplasm. is mainly stored in the lower hemi- 

 sphere, and that development is therefore more active in the upper. 

 The polarity of the ovum is accentuated by other correlated phe- 

 nomena. In every case where an egg-axis can be determined by the 

 accumulation of deutoplasm in the lower hemisphere the egg-nucleus 

 sooner or later lies eccentrically in the upper hemisphere, and the 

 polar bodies are formed at the upper pole. Even in cases where 

 the deutoplasm is equally distributed or is wanting if there really 

 be such cases an egg-axis is still determined by the eccentricity 

 of the nucleus and the corresponding point at which the polar bodies 

 are formed. 



In vastly the greater number of cases the polarity of the ovum has 

 a definite promorphological significance ; for the egg-axis shows a 

 definite and constant relation to the axes of the adult body. This 

 relation is, it is true, somewhat variable in different animals, yet the 

 evidence indicates that as a rule it is constant in a given species. It 

 is a very general rule- that the upper pole, as marked by the posi- 

 tion of the polar bodies, lies in the median plane at a point which is 

 afterwards found to lie at or near the anterior end. Throughout 

 the annelids and mollusks, for example, the upper pole is the point 

 at which the cerebral ganglia are afterwards formed ; and these 

 organs lie in the adult on the dorsal side near the anterior extremity. 

 This relation holds true for many of the Bilateralia, though the 

 primitive relation is often disguised by asymmetrical growth in the 

 later stages, such as occur in echinoderms. It is not, however, a 

 universal rule. The recent observations of Castle ('96), which are 

 in accordance with the earlier work of Seeliger, show that in the 



1 The third plane is in this case not precisely at the equator, but considerably above it, 

 forming a " parallel " cleavage. 

 '-"55- P- .>o. 



