56 PROFESSOR E. RAY LANKESTER. 



The four chief types of Egg-cleavage and of Gastrula-forma- 

 tion. — According to Haeckel we may distinguish in the 

 processes of egg-cleavage and the immediately subsequent 

 phenomena of development which result in the assumption 

 of the essential gastrula-form — four types, one primitive (the 

 true palingenetic series of changes), whilst the other three 

 are cenogenetic modifications of the process brought about 

 by variations in the quantity and mode of admixture of the 

 food-material. In each type five stages of development may 

 be distinguished, leading as far as the completion of the 

 Gastrula form — namely, the Monerula or fertilised egg after 

 the loss of its ovarian nucleus or germinal vesicle ; 2, the 

 Cytula or egg with newly formed cell-nucleus ; 3, the 

 Morula or mulberry form (Polyplast), a spherical agglomera- 

 tion of simple equi-formal cleavage-cells ; 4, the Blastula 

 or blastosphere, a hollow one-cell-layered vesicle, formed by 

 the accumulation of fluid within the Morula ; 5, the Gastrula, 

 a simple two-cell-layered sac, whose cavity, bounded by the 

 two primary germ-layers, opens to the exterior by the 

 Archistom (or blastopore). 



To the five stages belonging to the simplest type (that 

 which he considers strictly palingenetic) Haeckel gives the 

 prefix ' archi.' We thus have a series designated as fol- 

 lows : — Archimonerula, Archicytula, Archimorula, Archi- 

 blastula, Archigastrula. To the second type the prefix 

 ' amphi ' is assigned, and we thus have a corresponding series 

 leading to the Amphigastrula. The third type have ' disco ' 

 as their distinctive name-component, whilst ' peri ' marks 

 the fourth group. We shall take these into consideration 

 one by one, merely remarking to begin with that the Archi- 

 blastic and Amphiblastic series correspond to two modifica- 

 tions of what have long been called Holoblastic eggs ; whilst 

 the Discoblastic and Periblastic series are two extreme 

 forms of Meroblastic eggs. Haeckel is careful to point out 

 that though these four types can be conveniently erected as 

 a means for arranging our conceptions, yet that they really 

 are not sharply cut off from one another, but as we should 

 expect there really are transitions in blastoderm-formation 



stance of the sarcode mass. It is, however, no doubt most conducive to a 

 clear appreciation of the facts of the case to regard it as belonging to a 

 group of structural phenomena which are to be distinguished from the ulti- 

 mately resulting embryonic phenomena classed as heterotopy, heterochronous 

 hypertrophy and heterochronous atrophy, and may be designated ' matri- 

 ficial ' (like artificial). These matrifacts are often clearly ,the antecedents of 

 embryonic cenogenesis. Such are besides the added food-material, the egg- 

 envelopes. Structural phenomena proper to the protoplasm of the egg-cell 

 itself may be called ' ovificial' or ' ovifacts.' 



