INTRODUCTION 7 



poles. We call this kind of cleavage total unequal cleavage, 

 and group together as holohlastic eggs the forms belonging 

 to this type and those previously mentioned. The unequal- 

 walled blastula that has arisen through total unequal seg- 

 lentation can in its further progress lead to the formation 



a gastrula by invagination, but in this case the gastrula 

 "avity will be relatively shallow, corresponding to the small 

 size of the segmentation cavity. 



In some other cases, on the contrary (e g., in some 

 Annelids), after the conclusion of the process of total unequal 

 cleavage, there is formed a blastula in which the cleavage 

 cavity is reduced to a minimum. Accordingly there results 

 from segmentation a more or less solid cell-mass (sterroblas- 

 tula), in which we can distinguish a portion composed of large 

 entodermic elements rich in food-yolk from an ectodermic 

 portion consisting of small cells. The latter is placed upon 

 the former like a small cap in the region of the animal pole. 

 Here gastrulation by invagination cannot take place ; but the 

 gastrula-stage is formed by the growth and consequent 

 increase in size of the cap-shaped ectodermic part, whereby 

 its edges push themselves more and more over the entodermic 

 mass, so that finally the latter is entirely included within 

 the ectodermic sac. We designate the solid gastrula-stage 

 arising in this manner as a circumcrescent or epiboUc gastrula 

 (sterrogastrula) . By this means a gastrula cavity is not 

 formed primarily, but arises only secondarily as a fissure in 

 the entodermic cell-mass. The edges of the spreading ecto- 

 dermic layer must be regarded as the blastopore, which 

 accordingly is filled by a so-called yolk-plug. 



The presence of large quantities of yolk matter in the 

 region of the vegetative half of the egg presents obstacles 

 to the progress of cleavage in that region. The accumula- 

 tion of large masses of yolk can go so far that this portion 

 of the egg does not at first take part in the segmentation ; 

 but only a small portion, situated in the vicinity of the 

 animal pole and consisting principally of formative yolk, 

 is divided into blastomeres. Such, which undergo only a 

 partial cleavage, are known as meroblastic eggs, in contra- 

 distinction to holoblastic ones. There is developed in this 



