<;KI;MINAL LAY EMS 



27 



When the whole ovum is involved in segmentation the egg is termed holoblastic. This variety 

 of cleavage occurs in all eggs which are either alecithal (see p. 9) or moderately telolecithal, though 

 in the latter the division is very unequal, as, for instance, in the common frog. In exaggeratedly 

 telolecithal eggs like those of some fishes, birds, reptiles, and monotremes among mammals, the 

 cleavage is confined to the animal or protoplasmic pole, and they are then said to be meroblastic. 

 In all holoblastic eggs, except in the case of the mammal, the whole ovum is utilised for the 

 formation of the embryo ; while in meroblastic eggs only a small portion forms the embryo, 

 the remainder becoming the yolk-sac and the egg-membranes. The egg of the placental mammals 

 is holoblastic, yet the later stages correspond with those of meroblastic ova : hence it is a com- 

 monly accepted opinion among embryologists that the mammalian ancestry had large yolk- 

 laden eggs. This opinion is strengthened by the fact that the most primitive mammals the 

 Monotremata have eggs like those of reptiles. We are therefore justified in believing that in the 

 descent of the mammalia the yolk was lost when the egg came to be retained in the uterus 

 and established nutritive relations with the maternal tissues, but that it has retained in 

 some respects its ancestral mode of development. It must be added, however, that, while this 

 is the view of most embryologists, others (e.g. Hubrecht) maintain the opinion that the 

 mammalian ovum inherits its mode of development from ancestors with telolecithal holoblastic 

 eggs, like those of the present-day amphibian forms. 



FORMATION OF THE GERMINAL LAYERS. 



The youngest known human ovum (fig. 93, p. 65) is already considerably 

 advanced beyond the stage of the blastocyst. The resemblance between the 

 early stages in man, apes, and monkeys is very close. In certain particulars they 

 differ from the early stages of any other mammalian form except Tarsius spectrum. 



This creature has been commonly placed among the lemurs, but Hubrecht has shown from 

 embryological evidence that it is more closely related to the apes and to man, and he has 

 proposed to limit the order Primates so as to include only man, the apes and monkeys, and 

 Tarsius. These form embryologically a group by themselves among the mammals, and it is 

 now possible, thanks to the work of Selenka and of Keibel on the apes, and of Hubrecht on 

 Tarsius, to combine the data for the lower Primates with the data collected for man first by 

 His and Graf v. Spee, then by Keibel, Kollman, Peters, Eternod, Mall, Minot, and others, so as 

 to obtain a fairly complete, if in some points still hypothetical, picture of the early history of 

 the primate ovum. 



The earliest phases have been observed only in Tarsius, 1 but from later 

 resemblances there are cogent reasons for believing that, except in one or two 

 particulars, these phases may be taken 

 as representing what actually takes place 

 in the development of the human ovum. 



Formation of the entoderm. 

 We shall begin with the stage of the 

 blastocyst, represented in fig. 37. The 

 trophoblast forms a continuous layer over 

 the inner or formative cell-mass, which 

 projects as a rounded knob into the cavity 

 of the vesicle. Compared with the blasto- 

 cyst of the lower Amniota, this mass of 

 cells is as it were projected (invaginated) 

 into the interior of the vesicle instead of 

 being spread out on the surface (fig. 38). 



From the inner face of the inner cell- 

 mass a layer of cells becomes split off, 

 which is generally called the primitive or 

 yolk entoderm (lecithophore of Van Beneden) 

 (fig. 39, A). Contrary to what happens in most lower mammals, this entoderm 



1 A. A. W. Hubrecht, ' Furchung und Keimblattbildung bei Tarsius spectrum,' Verhandelingen der 

 Koninklijke Akademie van Wetenschappen te Amsterdam, viii. 1902. 



fern. 



"" troph- 



FIG. 87. BLASTOCYST OF TAKSIUS 

 SPECTRUM. (After Hubrecht.) 



fcm., formative cell-mass; troph., trophoblast. 



