614 INVERTEBRATA CHAP. 



posterior crescent contain only slaty-blue, yolky substance, but those 

 of the anterior crescent contain a rim of clear substance in front and 

 slaty-blue behind. The cells of the animal region of the egg, corre- 

 sponding to the ventral hemisphere of the embryo, divide into 

 similar daughters, all alike ; in front, however, the two most anterior 

 just surmount the equator. They all consist of clear substance and 

 yolky material, but the clear substance is much more prominent 

 than the yolk. At this stage, too, they are all columnar, and the 

 cells of the vegetative pole broad and flat ; at a later period, as we 

 shall see, the shapes of the cells of the two poles become interchanged. 

 The cells have separated from one another internally so as to give rise 

 to a blastocoele, and the egg has now become a blastula (Fig. 445, A). 



It is now possible to see clearly the exact regions of the embryo. 

 The cells of the animal hemisphere, or antero-ventral portion of the 

 embryo, give rise to ectoderm. Those two, however, which surmount 

 the equator in front when the egg is viewed from the dorsal aspect, 

 i.e. from the vegetative side, give rise to the anterior portion of the 

 neural plate. The blue cells of the vegetative hemisphere give rise 

 to the endoderm, but the most anterior cells of this hemisphere, 

 consisting of clear and blue substances, form the dorsal lip of the 

 blastopore, corresponding to the edge x in Amphioxus. They are 

 called chorda-neural cells (ch.n, Fig. 443, D) by Conklin, because, 

 like the cells forming that edge in the embryo of Amphioxus, they 

 give rise by further division to both neural plate cells (ectoderm) and 

 notochordal cells (endoderm), the former being constituted by the clear 

 and the latter by the blue portions of the cells. The transverse band 

 of yellow cells gives rise not only to the longitudinal muscles of 

 the tail of the larva but also to what Conklin calls "mesenchyme," 

 i.e. anterior mesoderm to which the muscles and genital organs of 

 the adult owe their origin. 



A register of the whole thirty -two cells shows, therefore, fourteen 

 ventral ectoderm cells, two neural plate cells, four chorda-neural cells, 

 six endoderm cells, and six yellow mesoderm cells. The blue cells 

 form a broad band in front which gives rise to the pharynx of the 

 larva, and to a tongue, projecting backwards between the horns of the 

 yellow crescent, which is the rudiment of the tail endoderm. 



By the next cleavage sixty-four cells are produced. Turning 

 first to the animal hemisphere, we find that the spindles of the 

 dividing cells are directed in various ways, but the result is to 

 produce a coherent sheet of twenty-eight similar ectoderm cells. 

 The neural cells divide so as to produce a curved transverse line 

 of four cells, and to the outer edges of this crescent there is added, 

 on either side, a cell from the rest of the ectoderm which also con- 

 tributes to the formation of the nervous system; so that, in this 

 way, a crescent of six neural cells is formed. 



Coming now to the vegetative hemisphere, we find that the chorda- 

 neural cells have divided into anterior and posterior daughters. 

 The four anterior are also neural cells and enter into the formation 



